In my area, it sees like some of the driveways have turned into roads because of overly helpful contributors.
Fixing it in OSM fixed it slowly in all drivers navigation systems and after a year no deliveries were lost anymore
They can look in some government maps, depending on the licenses.
[1]: https://help.openstreetmap.org/questions/615/can-google-maps...
But they do not make wide use of OSM data, because one of the terms of the license is attribution, of which there is none on any Google product I have seen.
I would guess they are big supporters of OSM as a hiring thing - by supporting an opensource project, you will get young engineers interested in mapping, some of which will then go work on Google Maps.
It also helps any antitrust cases if Google isn't the only game in town, which was the case for many years.