Some visa issues are very complicated. If I go to country A first, don’t leave the airport, and transfer to country B, do both countries A and B need visas?

This depends on which country’s passport the passenger holds and the specific regulations of country B on transit. For example, passengers holding Chinese passports are not allowed to leave the airport in Schengen countries and Russia (but the border inspection of country B, not customs. Customs checks items, not passports, visas and other documents, and visas are checked by border inspection). If the time does not exceed a certain time, you can transfer without a visa. In some countries, if there is an onward flight ticket departing from that country, you can apply for a transit visa on arrival and exit the airport. This was the case in Malaysia before, and the same is true in Ethiopia. With a Chinese passport and a flight ticket departing from Ethiopia, you can apply for a transit visa on arrival ( Many Chinese who go to other African countries use Ethiopian Airlines). But for the same Chinese passport, the United States, Canada and Mexico have different regulations. If you hold a Chinese passport and transfer from any country in the United States, Canada and Mexico, even if you do not leave the airport, you will still need a transit visa for the country's border inspection. These countries are not "a very small number of countries". Some travelers to South American countries choose the United States or Canada as a transit country (the air tickets are cheap and there are many route options). For another example, it is also a Schengen country. For Afghan passports, regardless of whether you leave the airport or not, you must hold a transit visa (or other types of Schengen visas) before you can transfer, just like the United States, Canada and Mexico for Chinese passports.