I think this will vary tremendously by individual kid, teacher and class. My kids did basically nothing outside of what was required by their classes. That said, I think their classes/teachers over prepared them- our school’s AP classes are open to all kids, and the volume/degree of preparation reflects this. By the last month or so before the exam, most teachers have made efforts to familiarize kids with the exam expectations, some generally and some quite explicitly. For AP history classes, the format expected for essays is important; we had one teacher that used that format weekly, from the start of the class (huge overkill, not a fan, made my kids nuts and was not useful for good writing otherwise, but I digress). Foreign language teacher used media resources (watching news, videos, etc) that were similar to what would be tested, but this was integrated into the class. Science and math teachers here probably did the least of this, but did at least make sure kids knew the format/timing of the exam. If the class is solid, they should be fine for the exam, in our experience.
My advice would be to ask the teacher what they recommend- some of them have a favorite review book and prefer the kids use that, or even use it in class. Also, I am pretty sure there are “official online resources” that are now made available to students when they register for APs, but we have no experience with that. The last thing may not be helpful, but our kids always had group chats set up with others in the class, and they frequently shared resources (I know AP world and physics C were classes where YouTube videos were shared). I can ask the kids if there is a specific class you are interested in.
