Three big caveats:
- None of this has run through court, so it’s not settled law.
- Not a lawyer.
- I’m speaking broadly about the PS3 emulator unless otherwise specified
In terms of the US, Emulators ARE kinda generally illegal - they re-implement copyrighted (and potentially still patented) hardware, and they do not do it in the ways that are explicitly allowed for reverse engineering, and they bypass or modify copyright protection mechanisms in doing so. That hits pre-DMCA laws, as well as DMCA requirements around DRM/Copyright Protection mechanisms. The older the system the less likely the hardware is to be encumbered with those restrictions, OR fall into the various exceptions carved out specifically for the emulation of older hardware. Plus, the actual owner of the rights regarding that hardware is less likely to intervene - do not underestimate how much of the state of emulation is due to companies just not reaching the point of caring, rather than actual legality.
This is basically the same set of laws that would hit you if you built a PS3 clone - there is a very, very specific set of steps required to be able to do that, and Sony could STILL sue you and force you to establish to the court you followed them to the letter, and even if you won, they could stop you from making units that look like PS3s, or using any of their trade dress, or the word PS3, or screenshots of games to promote it.