Does Google use GoLang for its backend? Why?

Some teams, yes. Its GO lang from GOogle

for some stuff yes. Not a fan of all the language fragmentation in software myself but here are some examples why https://go.dev/solutions/google/

A lot of the newer teams, like mine, use it. We have a ton of c++ that isn’t going away anytime soon though.

c++ and java mostly

Nope, at least not in Google Search world