I guess I don't know enough US teens or US college students then. 🤣
So if you have a group of 10 students it's statistically likely you'll have to use WhatsApp and can't use iMessage for a group chat.
For those at my company who get given phones they tend to be iPhone SEs as it makes it somewhat simple for them to choose rather than the wide range of Android. As I say probably about 2/3rds of the people I know from Engineers, project managers, office workers, accountants, music promotors, teachers, business owners, entrepreneurs, brewers, and pretty much all walks of life here in the UK have Androids. My girlfriend has an iPhone. My dad has an iPhone. My mum used to have an iPhone but now has a Samsung. The 3 people I go mountain biking weekly with one has iPhone the other 2 Android (WhatsApp group there then). The singer in my band has iPhone, the other 3 Android (FB messenger group). Not knowing anyone with an Android phone must be a very US thing. Maybe a money/social group thing too. You can pick up a lot more cheap and passable Android handsets.
I just googled and supposedly iOS/Android is pretty much a 50/50 split here in the UK (unverified data source). To me it seems less based on my experience. Maybe it's slightly higher than I think but I'd definitely say the majority don't.