How it works with connection flights?
For example, AMS-CDG-JKF, what kind of passengers will flight this route:
one from AMS to CDG and other to JKF or
one from AMS to CDG, other from AMS to JFK and other from CDG to JKF??
Also abour seat types - are there some difference between high density, standart and premium on longhaul routes (like CDG-JFK).
Connecting flights aren't implented in game.
Yes, seat type is important. Especially on long-haul.
In your example you will fly one set of passengers AMS-CDG and another set of passengers CDG-JFK. Connecting passengers are not modeled in the game currently. AMS-CDG and CDG-JFK flights are two completely different flights which are just operated using the same aircraft.
It used to be that seating comfort did not matter but in the current version will prefer the flights with better seats especially on long-haul routes.