Google's network tracks and manages network usage at the edge. I'm not sure if experts would say that was done "by borg" or not. On each host there is a "host enforcer" which manages flows to and from that host. You can read all about it at [1]. I don't think there are any publications suggesting that Borg uses network flow rate for scheduling.

1: https://research.google/pubs/pub43838/

For the most part, other services take care of network resources. Borg might have some host level network limits, but I haven't seen them used much.

Yes. Not directly Borg but another infrastructure component does.