Where does “бы” come from?

бы (Russian) comes from Old East Slavic бꙑ, from Proto-Slavic *by.

Ancestry of “бы”, step by step

Step	Language	Word	Meaning
1	Old East Slavic	бꙑ	—
2	Proto-Slavic	*by	—

бы — Etymology, Origin & Word History | Etymology Explorer