Overheating, screen goes dark

My iPhone 12 Pro Max, bought new in Oct 2020, is now toast. It’s been overheating so much - first while on charge, then while trying to connect 5G, occasionally in sunshine, but now I can’t even leave it on for a few minutes before it’s super hot and dark screen. Can’t use any apps, compose an entire email, open a web link, it’s nuts! Always protected, never wet, never dropped. Anyone else? Two Apple Store trips and they’re clueless. Also Face ID repeatedly crashes too.