TIL/remembered GNSS satellites have onboard atomic clocks. Makes a lot of sense, but still pretty cool. Something like this, I guess?

https://en.wikipedia.org/wiki/Rubidium_standard

Yes, either Rb, Cs, or H standards depending on which GNSS system you're using.

For the most critical applications, you can license a system like Fugro AtomiChron that provides enhanced GNSS timing down to the level of a few nanoseconds. There are a couple of products that do similar things, all based on providing better ephemerides than your receiver can obtain from the satellites themselves.

You can get AtomiChron as an optional subscription with the SparkPNT GPSDO, for instance (https://www.sparkfun.com/sparkpnt-gnss-disciplined-oscillato...).