microG is what does the talking to Google Play Services the networked services - instead of the Play Services the system components.
That allows for some flexibility, but not that much.
microG offers a gradual off ramp from networked Google services and apps to find functions (Cloud messaging, Location, Barcode Scanning) they’d expect on a Stock Google Android and not crash because they can’t find them.
Here’s the /e/OS docs: A simple guide to understand microG or a Video: microG explanation as video or imo a very telling API implementation overview.