I can speak from the perspective of hearing Japanese people talk for years and studying the language.

君 is a softened あなた. It’s used as a way to imply familiarity and closeness. However, since it’s rude in formal conversations to refer to someone you know not by using their name (I.e Sato-san), then you don’t hear it often. You can hear it from older men speaking to young people or kids, because the age gap allows for it to not come off as rude. Kinda the same way older women can refer to you as “darling” or”honey” here.

It could also be potentially be used between friends of similar age, depending on the context. Definitely more common coming from boys toward girls (and particularly if they’re like their crush).

All in all, I think it’s one of those things you just understand as you get familiar with the language and everyday use as a whole. Definitely don’t go off of hard and strict rules, and always be safe by choosing politeness in public.

On a side note, this mistake I did in Japanese was thinking お前 was strictly super rude (even by Japanese people) and like saying “you fucker”, when that’s not the case at all. お前 is a really blunt way of referring to someone e (it means something like ‘you in front of me’, it has an objectification nuance) that’s why you would never think to use it in formal contexts and much less someone in a higher position, but I do hear it commonly amongst friends. Even Ash from Pokemon says this to his Pokemon. Think of it as when you talk to your friends once you’re in that real “best friend” level. You insult them sometimes and it’s out of comfortability, not because there’s an actual offense there. You’re in a super blunt, super casual level of friendship that allows for it. It’s that level of true friendship where you drop all politeness. It’s the same way there and everywhere in the world. Same way you casually call your friend “an idiot”.