With GNSS+PPS and a hardware timer latch you can easily sync internal microcontroller timers to 2 to 3-digits nanoseconds against the global standard with a tightly controlled loop. But cannot get better than the PPS signal itself (roughly 30-100nS).

Everything below needs that a huge amount of engineering. CERN&friends developed the "White Rabbit" (https://ohwr.org/projects/white-rabbit/) system for this. This allows sub-Nanoseconds syncing of netwoked devices.