pressure cabs use the air pressure in the venturi and the ram air pressure in the intake to act as a metering force accross a bellows. This is in effect a way of mass flow metering as the carb leans as the intake air pressure drops (and density) when you climb to altitude. they are generally auto lean carbs although they have a mixture control too.
I dont work in GA but i think most light GA engines use float carbs.
You do get mass airflow meters. thats how fuel injection on a car engine is done but aviation engines tend to keep it simple.
Ill try to find some more detailed info on pressure carbs and pressure-injection carbs, its been a while since ive looked into them.
Stu.