So if for example someone coming from Spain with Qatar airways to BKK and book separate a flight from BKK to Chiang Mai with Viet air, maybe 2 hours after arrival... (just examples) there should be only 1 immigration stop in immigration, right? As you said, makes no sense to do 2x Immigration.
at the moment I didn't see direct flights from Germany to Chiang Mai for example, all go through BKK k or many hours of waiting in Singapore with scoot airline.