What is exactly Google Play System Update buried deep in the settings that can be manually updated and usually has a date of say, eg, May 1. Is it part of Project Treble from Google to decouple parts of OS from full system upgrades and make more Modular updates possible?
Yeah, project mainline to reduce the impact of full system upgrades.
You can read about it here: https://www.esper.io/blog/what-is-project-mainline