Where does “Prydeiniwr” come from?

Prydeiniwr (Welsh) comes from Welsh Prydain, from Middle Welsh Prydein, from Proto-Brythonic Pritanī, from Proto-Celtic *Kʷritanī, from Proto-Indo-European kʷer- — to do; to make; to build.

Prydeiniwr (Welsh): Briton, Britisher (modern inhabitant of the island of Britain)