As a google employee I think by far the most likely outcome of that scenario (where a google engineer modifies a piece of OSS to make it significantly better) w

As a google employee I think by far the most likely outcome of that scenario (where a google engineer modifies a piece of OSS to make it significantly better) would be that engineer using a lightweight process to get permission to contribute to that project (if it’s not already white listed for contributions) and then sending the improvements to be merged upstream. There are many, many open source contributors at google and there is an entire group of people staffed to work on open source, among their responsibilities is to make it easy for google engineers to contribute to open source projects (both in their spare time and also as part of their daily work).