Google's CI system is proprietary, but learning how a CI system should work is valuable. Google's code review system is proprietary, but learning how code review should work is valuable. Repeat for every proprietary system Google has.

When I left Google for a startup, all the tools I took for granted didn't exist. But because I knew how things could and should work, I was able to apply open source and commercially available tools to improve software development practices at my new employer.