What is the legality of emulating proprietary hardware like this?
https://www.dualshockers.com/playstation-classic-emulator-pc...
Piracy of the PS4 would get their attention.
And I'm sure it's possible to find some game demos as well.
I can tell you that it is explicitly illegal to have the files you load into the emulator to play a certain game if you haven't purchased a copy of that game yourself.
There are not—as far as I'm aware—any laws against watching copyright infringing materials; intentional possession is the entirety of crime.
What you do with the copy is irrelevant.