Where can I post to receive tech support for anything outside of Lemmy?

You can try posting in !techsupport@lemmy.world.

I would start by checking if there's a relevant community to the thing I need help with, for instance there are already Samsung and Android communities, I guess there's one for Apple too, I would assume that for now you can make tech support posts in any of them.

If the thing you need help with is doesn't yet have a relevant community then posting in tech support communities like the one the other commenter linked might be your best bet.