All candidates are chosen by the Democratic Front for the Reunification of Korea, the single coalition to which all members of parliament and all parties in parliament, as well as some civic organizations, are a part, with one candidate per single member constituency. This is done in a pre-election meeting which is controlled in practice by the Worker's Party which is the real one party that runs everything in North Korea.
The actual elections are non-secret approval votes for the single candidate with implied threats of retribution for voters who decline to support that candidate, without regard to that candidate's formal party affiliation.
Direct evidence from North Korea on the ground from unofficial sources is hard to come by, some much of what follows is an educated reasonable inference from what we know, because that is the best we can do.
The Worker's Party gets 8/9th of the seats which may be a traditional formula kept in place since the early days of the Democratic People's Republic of Korea (DPRK) when minority parties were co-opted while the Communists were still in the process of securing total control, and then kept in place for appearances sake after the other parties had been completely defanged.
The Social Democratic Party and the Chondoist Chongu Party were the two other parties, in addition to the Worker's Party, that existed in Korea prior to the formation of the Democratic People's Republic of Korea (i.e. the North Korean government), and haven't be entirely abolished, although they are controlled by the Worker's Party in practice.
The Social Democratic Party:
was formed on 3 November 1945 as the Korean Democratic Party by a
diverse group of medium and small entrepreneurs, merchants,
handicraftsmen, petite bourgeoisie, peasants, and Christians. The
party's founders were motivated by anti-imperialist and anti-feudal
aspirations, and aimed to eliminate the legacy of Japanese rule and
build a new democratic society.
In the 1980s, the regime experimented with using this party as an outlet for mild, controlled, peaceful dissent, but that experiment had been abandoned by the 1990s. This party is now used mostly for foreign propaganda by North Korea:
The Korean Democratic Party was renamed the Korean Social Democratic
Party in 1981. Since then, the party has been used in North Korean
propaganda targeting foreign sympathizers. Because of the ostensible
social democratic ideology, which is intelligible to foreigners, the
Social Democratic Party is used in such propaganda much more than the
other legal minor party, Chondoist Chongu Party. In the 1990s, the
KSDP published periodical magazines in Korean and English. These
magazines sought to simultaneously convince foreigners that North
Korea has a multi-party system with independent parties but also that,
paradoxically, minor parties in North Korea support the Workers' Party
of Korea without reservation.
Nominally, the party seeks to establish a social democracy befitting
Korea's historical conditions and national characteristics. The
party's motto is "independence, sovereignty, democracy, peace and the
defence of human rights".
The Condoist Chongu Party was a grass roots Korean nationalist traditional Korean religion movement, mostly among peasant farmers that formed a political party as well to resist the influence of Christian missionaries. It was a close rival to the Communist Party before the Democratic Republic of Korea was declared whose anti-communist leaders and activists were purged in 1948 and again in 1958, leaving it an empty and impotent shell that was kept in existence so that the DPRK can claim that it is a multi-party democracy. In officially secular and communist North Korea, its party line has grown incoherent and has largely been silenced.
The Chongyron Party is named after a Korean ethnic enclave of Japan (many residents of whom were born in Japan but are still not recognized as Japanese citizens), and is intended to represent the interest of members of this expatriate Korean community in Japan who feel ties to North Korea. The size of this delegation is probably tied to estimates of the population of this enclave in Japan, relative to the North Korean population overall. One can analogize these seats to the delegates that U.S. major political parties give to members of their party living abroad, for the sake of an appearance of broad inclusiveness (and as a way for North Korea to remind itself of the grievances it bears towards Japan).
It isn't entirely clear what role is played by the two independent members of the parliament or how they are selected. Local elections in rural North Korean villages are non-partisan. There are oblique suggestions if you read between the lines (e.g. in a Wikipedia list of non-partisan national legislators) that the independent seats are awarded to non-partisan local elected officials who are distinguished in their support of the Worker's Party or its policies, despite not being party members (perhaps because their villages were too small to have a proper Worker's Party committee at the local level).