The list of things I can think of is:
* Linux
* LLVM
* Webkit/Chrome (which they have done the majority of contributions to for a long time)
* Java & a little bit of Python
(disclaimer, used to work at google a long time ago)
My personal experience was I never used any OSS code (that wasn't Google Open Sourcing its own code) except Linux & LLVM.
It definitely didn't feel meaningful to the company besides the ones I called out.
The people meaningfully benefiting from open source are the people and companies on the margin, not the biggest tech companies in the world.
Lots of Linux contributions for Rust drivers