Where does “въ” come from?

въ (Old East Slavic) comes from Proto-Slavic vъ(n), from Proto-Indo-European h₁n̥, from Proto-Indo-European h₁én — in.

Ancestry of “въ”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	vъ(n)	in, inside, within; in, into, inside; at
2	Proto-Indo-European	h₁n̥	—
3	Proto-Indo-European	h₁én	in

въ — Etymology, Origin & Word History | Etymology Explorer