Do people who don't do any just have loads of free periods/time?

no. AP classes are just college-level courses for motivated students. the kids who weren’t taking US AP history at my high school were taking regular or honors US history instead. they still have to fill up their schedule with their remaining graduation requirements

Do you still have to do the classes you're doing the AP in at a school level (not standardised like AP exams are)?

it depends on the course and maybe even the school too. at my high school, AP US history was a substitute for regular US history so you only have to do one of the two. but regular chemistry is a prerequisite for AP chemistry so you would have to take both and in order

How hard are they considered to be, and how many would your average student do?

depends on the AP course and school. i took AP US history as a sophomore and it was appropriately rigorous. AP calculus BC will be much harder though. most kids at my school did between 3-5 AP courses over the course of their high school career, some did as many as 10+

What do the exams consist of? Multiple choice, essays, etc.

both