transit visa rules for air travel? [closed]

I have flown to a handful of countries in the past, sometimes requiring transfers. After reading some articles, I am confused about the rules and regulations of air transfers regarding to immigration.

Let's start with my background. I'm from Hong Kong, which only has "international" (not the literal sense but in the immigration sense, i.e. flights from HKG to other parts of China are considered "international" flights"). Passengers transferring in Hong Kong within airside do not pass through immigration, hence don't need a visa (except for very few nationalities which require "airside transit visa"). Furthermore, a ferry terminal (called the "Sky pier") is integrated within the airside so that passengers from other parts of the Pearl River Delta can directly take a flight from HKG without passing through HKG immigration.

All the different airports I have travelled to include domestic zone and international zone if I remembered correctly, unless it is in a small territory which does not have domestic flights, which mean I can transit between international flights without passing through immigration. However, this may not be true and may have exception, which may cause troubles.

My first international travel by air was in 2009, which I flown from Hong Kong to Bulgaria via Germany with about 6 hours layover for an international competition. As excepted, we stayed in airside in transit, as a result, we did not pass Germany immigration.

However, things were not as smooth as before in a family travel in 2010, which my family flown from Tianjin to Singapore via Malaysia with half-day layover. When checking in the first flight, we were instructed to go through immigration in Malaysia, then check in the connecting flight there. I passed through immigration with normal visitor terms because I held an HKSAR passport, but one of my family members had to go to get a transit visa on arrival after getting off the plane at Malaysia because she held a mainland PRC passport, which required applying for a visitor visa, but visa on arrival could be obtained for transit as she had a Singapore visa. We finally ended up having Malaysia stamps on our passports.

So what are the general rules regarding transfers? If I want to go from A-B-C by air, all different countries, stopping only inside the airport at B, in what circumstances I must get a visa for B? I have found articles on the Internet telling us to avoid transiting in the US because we have to pass through immigration and hence visa, and some even tell us to avoid stopping flights.

• If I go from A-B-C, transiting between different airlines, do I have to go through immigration at B?
• If I go from A-B-C, transiting between the same airline, do I have to go through immigration at B?
• If I go from A-C, but the plane stops and embarks/disembarks passengers at B, do I have to go through immigration at B?
• If I go from A-C, but the plane just stops at B as a technical stop/fuel stop, do I have to go through immigration at B?