I don't know much about the practicalities of unofficial diplomatic relations between China and countries that recognize Taiwan, but what I do know is that China has little trouble doing business with them, e.g.
China’s pragmatic attitude did not prevent the country from doing business with Panama even before formal diplomatic ties were established between the two countries. The same goes for China’s business relations with Guatemala and Honduras, two countries that are still close to Taiwan. Indeed, from 2006 to 2016 Chinese exports to Guatemala and Honduras increased by 300% and 750% respectively. [...] For example, China has become the second exporter of vehicles to Guatemala and Honduras with a market share of 11.75% and 14.75% respectively.
Trade also goes the other way, e.g. Guatemala exports to (mainland) China, mainly foodstuffs. (Not surprisingly they have trade a deficit with China.)
Guatemala and Honduras are among the countries that recognize Taiwan. Guatemala and Honduras don't recognize China. They don't have PRC embassies but ROC (Taiwan) ones. Taiwan for example has a Taipei Economic and Cultural Representative Office in the US. I'd venture a guess that China has something like in (e.g.) Guatemala and Honduras, but I can't find any details... besides what they were called 10 years ago. FAO has this pretty dated document (2009) on alternative arrangements for countries with no diplomatic relations with China:
As you can see, in some cases, the Chinese "trade development office" even accepts visa applications.