Depends if you mean the Yaris GR or the GR Yaris
The Yaris GR is just a normal Mk4 Yaris Hybrid with big rims, while the GR Yaris is the front half of a Yaris Mk4 and the rear half of a Corolla, with the world's most powerful production 1.6 litre inline 3 cylinder engine, and one of the lightest AWD drive trains. It was built because Toyota wanted to completely obliterate all the competition in the World Rally, which requires them to actually make and sell the car they're basing their rally car on, but ironically because of the coronavirus they never got to field it!
It's a damned shame as I would have loved to see how it fared against the competition!
It's probably one of the best cars you can buy for hooning around back roads and general driving enjoyment without needing a 2nd mortgage, and is likely the last one of it's kind before everything goes EV.