Unlike The Wii U, essentially 100% of everything Wii related works, no relicensing bs etc.
Same as Wii, Phat PS3, and PS2. It's hardware-based BC which gets around the licensing thing. Phat PS3 had an emotion engine chip on it as example. Wii had the gamecube components on it.
The one I don't know how they get around it is PS3 + PS1 software emulation.