Migrant transit country

It depends on what kind of passport the passenger holds and the specific regulations of country B on transit. For example, passengers with China passports are not allowed to leave the airport in Schengen countries and Russian countries (but the border inspection in country B is not just the customs, but the goods are inspected by the customs, not passports, visas and other documents, but the border inspection). If the stay time does not exceed a certain period, the transit can be exempted. In some countries, they can apply for a transit visa and leave the airport if they have a continuation ticket from that country. For example, Malaysia used to be like this, and so did Ethiopia. With China passport and Ethiopian air ticket, you can go to landing visa (many China people go to other African countries by Ethiopian Airlines). However, the United States, Canada and Mexico have different regulations on China passport. Holding a China passport, you need a transit visa to transfer from any country in the United States, Canada or Mexico, even if you don't leave the airport. These countries are not "very few countries", and some travelers to South America choose the United States or Canada as transit countries (cheap air tickets and many route choices). For another example, as Schengen countries, Afghan passports are required to have transit visas (or other Schengen visas) before connecting flights, regardless of whether they leave the airport, just like American, Canadian and Mexican passports to China.