IIRC, they’re legal as long as they don’t explicitly distribute any of the copyright owner’s own code or files. That’s why, for example, PCSX2 requires you to dump “your own” PS2 BIOS and doesn’t provide any itself. Because PCSX2 doesn’t distribute the PS2 BIOS and because its way of talking to the BIOS doesn’t copy the source code, that emulator is in the clear.

Some modern emulators (ex. Ryujinx) don’t even need BIOS files (or whatever they’re called on Switch) to be able to run games. But they also don’t use Nintendo’s original code to run the game.

Take all this with a grain of salt. I’m saying all this from memory.