The datacenter OS doesn't have to be the same as the developer OS. At my work (of similar scale) the datacenters all run Linux but very nearly all developers are on MacOS
MacOSX is a popular choice for dev boxes (if I understand correctly, they provide some pretty good tooling for managing a fleet of machines; more expensive hardware than a Linux dev machine fleet, but less DIY for company-wide administration).
... but Google solves the "A Linux fleet requires investment to maintain" problem by investing. They maintain their own in-house distro.
Not really, it is just a well known outside distro plus internal CI servers to make sure that newly updated packages don't break things. Also some internal tools, of course.
Relative to what the rest of the world does, that is maintaining your own in-house distro.
It's downstream of Ubuntu (unless that's changed) but it's tweaked in the ways you've noted (trying to remember if they also maintain their own package mirrors or if they trust apt to fetch from public repositories; that's a detail I no longer recall).
