While I'd prefer not to think about all this and just code, I appreciate that there is an existing policy instead of a bad surprise in a few years.
The current policy, which is 100% public (https://opensource.google/docs/iarc/), is basically "let's assume that we own everything except what we don't, and let's figure things out before it's too late".
If you contribute to an existing open-source project on github, this is basically a non-issue and the answer to who owns the copyright has practically no effect. There are pre-approved licenses and a few banned licenses (agpl, wtfpl, public domain, etc), as well as pre-approved CLAs with major entities (Python, Mozilla, etc).
I think this matters in two cases:
1/ you want to start a new open-source project, you don't want google to have copyright because you think that a good open-source license will not protect you/your work from being stolen (!).
2/ you want to start a side project and maybe one day you'll turn that into a business, and you don't want to be Pied Piper vs Hooli.