Go to https://github.com/google, but sort by language:

- C: https://github.com/google?q=&type=&language=c

- C++: https://github.com/google?q=&type=&language=c%2B%2B

- Go: https://github.com/google?q=&type=&language=go

- Java: https://github.com/google?q=&type=&language=java

Google's style guides: https://google.github.io/styleguide/

Hand-picked: https://source.chromium.org/chromium, https://android.googlesource.com/, https://fuchsia.googlesource.com/

Filter by subject: https://opensource.google/projects/explore/featured

Also you can do the same for https://github.com/microsoft and https://github.com/facebook