If you make a copy of your Wii U BotW ROM (something that's possible but I've no idea how to do lol) it's legal to then use that with the latest Wii U Emulators. It would be illegal to use anyone else's ROM, though, and Switch games cannot yet be emulated (as far as I know).
So, even if you own the Switch version, that would be useless to you when attempting to play it on the PC. The only strictly legal means of playing BotW on the PC right now is via the use of your own personal Wii U version BotW ROM with an emulator.
The reason emulation is often said to be illegal is that the vast majority of people who emulate download ROMs from others and don't use a copy of their own, legally purchased software. If you do, though, using it with an emulator that is built from the ground up without source code from the original hardware is perfectly legal... that's just not something most who emulate actually do lol
