The hangers were airlocks, they just had a soft shoeld to hold atmosphere under normal operation (and cheaper production costs). There were doors, they were just open for most scenes they were used in, and there were a few where the shield failed and air was lost while the interior doors were closed. I’m sure the idea was applied to man-sized entrances, as well, but no need to show a shield when you’re attached to a corridor from a different habitat.

And, of course, there were the bulkhead doors which could be closed in an emergency, which could turn any section into an airlock.

…and all of that ignores just how long any hole the size of a hallway would take to significantly depressurize a vessel the size of the Enterprise.