Where does “ꜥrw” come from?

ꜥrw (Demotic) comes from Egyptian -w, from Proto-Afroasiatic -u.

Ancestry of “ꜥrw”, step by step

Step	Language	Word	Meaning
1	Egyptian	-w	Used to form the masculine plural forms of nouns...
2	Proto-Afroasiatic	-u	—