Elections for the NPC seats are (largely) indirect. Additionally the wider NPC has less power in practice than in theory. Its members don't get a paycheck unless on the Standing Committee. Regular members can give speeches to the NPC but they hardly ever exercise this right etc. Thus, it's been generally described as a rubber stamp.
As for the lower-level elections, which elect the electors for the NPC as well, these are in theory open, but in practice are hardly ever won by non-CCP approved candidates. The NYT has a vignette (albeit from a decade ago) of how intimidation against non-CCP-approved candidate works in practice. Their supporters get urged to withdraw by CCP officials, and if they don't do it, they get further harassed by phone and otherwise, their social media access (Weibo etc.) cut inexplicably, etc. Lack of any (real) foreign observers to these elections means that in the end who counts the votes is what matters. As e.g. in that example, it was announced that the CCP-supported candidate had received 3-time as many signatures endorsing, so would be the only person on the ballot. Write-in candidates were actually possible in that location, so the non-CCP candidate's supporters did avail themselves of that method for voting for him, but at every step the election process was stacked against him in terms of getting his message out and even making it known that there was a non-CCP candidate. So he lost to the CCP one.
I'm not sure how up-to-date Wikipedia is on Chinese legislation, but campaigning does not seem legally protected even in theory:
The 1979 law was revised in 1982, removing the reference to the ability of political parties, mass organizations, and voters to use "various forms of publicity", and instead instructing that the "election committees should introduce the candidates to the voters; the political parties, mass organizations, and voters who recommend the candidates can introduce them at group meetings of the voters".
A bit more searching found this article on the Jamestown Foundation, which documents several more instances of similar occurrences as in the NYT vignette. FWTW, the official Chinese media describes Western-style elections in which several candidates appear on the ballot and with open campaigning as "circus-like" and a "train wreck" (that was in 2016, if you're curious). In contrast, the official verbiage is that the Chinese (one-candidate) approach
integrates process-oriented democracy with results-oriented democracy [...] and people's democracy with the will of the state.
One interesting point made in a 1989 paper was that no non-CCP party in the NPC had its own newspaper; although some of the more obscure (smaller) parties did have some less often published media--typically monthly; only the Revolutionary Kuomintang had a semi-weekly. (I'm not sure if that situation continues in the same manner.) Before the army crackdown, the Revolutionary Kuomintang also criticized the Tiananmen demonstrators in the harshest terms, compared to other DPGs (an acronym used by that paper to denote "Democratic Parties and Groups" in China, with representation in the NPC.)
After the Tiananmen events and the DPGs not wholeheartedly endorsing the repression right away, some rules were changed/tightened so that:
a person now can only join a DPG with the approval of the CP secretary in his or her unit.
I'm not sure if that is still the case to this day, but it gives some idea of the CCP controlled these other parties. As the paper comments (elsewhere) such rules are meant to keep true dissidents [opponents of the CCP] out of these alternative parties (that get some NCP seats).
That practical rule was also mirrored by the official language in a 1989 publication that set out the [somewhat revamped] principles of the multiparty system in China:
In practicing multiparty cooperation in our country, it is imperative to uphold the leadership of the Communist Party [...] The leadership the CP exercises over the
democratic parties is political leadership, i.e. leadership in political principle, political orientation, and major principles and policies.
Again I'm unsure how things might have changed in this respect since 1989, but according to one survey discussed in that paper, the DPGs were also obscure to most Chinese:
three-quarters of respondents unable to make informed comments about them, neither were a majority of CP members.
Actually, things probably didn't change much in any substantive regards, because e.g. the 2012-2022 leader of the Revolutionary Kuomintang had nothing but bad words about any possible alternance in power or real party competition:
In a 2013 interview, Wan said that the Chinese people crave the growth and stability of one-party rule and that the West had a fixation with electoral democracy. “We once had more than 300 parties in the early stage of the Republic of China, and the consequences were rivalry among political parties and warlords, and national disintegration. China could never have obtained such brilliant economic success today if we followed that kind of political system.”
The highest number of NCP seats for a "party" other than the CCP currently belongs to the Jiusan Society. It's hard to call this organization even a political party given their membership and focus, see e.g. official description:
is composed mainly of senior and mid-level intellectuals engaged in science, technology, culture, education and medical and health work [...]
Jiusan Society members have fully used their advantages in various sectors and talents to carry out deep investigation and research, raise many important opinions and suggestions, and provide important references for the scientific decision-making of the CPC and the government.
Thus one might say that alternative parties have been further de-emphasized in the Xi Jinping era. The largest of them seems to function as a scientific advisory group to the CCP.
The 2nd largest non-CCP by number of seats is "China Association for Promoting Democracy". You'd think this would be more politically oriented given its name, but it seems to be a clone of the Jiusan Society [albeit with a narrower base], at least going by the (much more terse) descriptions I can find, namely that it:
mainly represents high-level intellectuals engaged in education and cultural publishing media.
As for any political positions of the leader of the latter (CAPD), not much can be found except that he:
urged CAPD members to rally closely around the Communist Party of China (CPC) Central Committee, and better play their roles as advisors and assistants to the CPC.
Anyhow, the broader appeal of such "elitist"/narrow-base "parties" is essentially self-limiting. Another article reflects that joining some these parties is difficult because of the high educational requirements:
it is not easy to join these groups. Many parties require their members to have certain academic qualifications and titles. For example, some require that prospective members have a master's degree, intermediate professional titles (equivalent to associate professor), or outstanding contributions and awards in their respective fields.
Probably an organization with some real party credentials is the 3rd one among non-CCP organizations (4th by number of NCP seats if the CCP is counted), namely the CDL. Interestingly they even had a newspaper during the 1940s and '50s (the Guangming Ribao), but lost control over it during the Cultural Revolution, and the newspaper was later [1980s] permanently placed under the formal control of the CCP.
But nowadays, other than the year of founding, little seems to distinguish the CDL from the other two organizations I mention above (CAPD and Jiusan Society) at least going by the quasi-official description by the Chinese news agency.
Founded in 1941, the CDL, with a membership of 348,000, mainly consists of intellectuals working in the fields of culture, education, science, and technology.
So, one might easily describe the first three of these non-CCP NCP parties as "intellectual/academic club times 3". (Maybe there's something interesting here to say about the party-wise fragmentation of such elites in China, but I've not seen any commentary in that direction.)
On par with CDL as number of seats is the CNDCA (China National Democratic Construction Association), which seems to be more like an interest group for entrepreneurs in real estate & construction. I'd also describe this as self-limiting in its appeal.
And one might think that the Chinese Peasants' and Workers' Democratic Party would have a broad[er] appeal and membership given their name that they trace their history back to the 1920s. But nowadays they seem to function as a medical association. And if Wikipedia's description is possibly in doubt, the official news about them also stresses that
CPWDP has played a full role in in-depth research, particularly in the medical services, population and ecological environment fields.
The two or three remaining parties (that aren't narrowly self-selecting based on education or work-field like the above) are all focused on Taiwan or the Kuomintang legacy. The Revolutionary Kuomintang is some kind of left splinter group of the KMT. (By the way, unlike in larger NPC, they are afforded most [non-CCP] seats in the CPPCC--a consultative body, but on par with CDL.) The Zhi Gong party is also historically related to ex-KMT warlords [and thereafter was based in Hong Kong]. The Taiwan Democratic Self-Government League is supposedly representing [some] Taiwanese [descendants].