How can I determine where network data is going? Activity Monitor always reports several kB or MB being transferred. I do not know where it's going or coming from. How can I find out?
IMHO the most convenient and simple solution is the Network Monitor of Little Snitch.
It monitors all outgoing and incoming network in real time and provides

fine grained traffic information, combined with powerful yet simple options for analyzing bandwidth, traffic totals, connectivity status and much more.

Network traffic filtered by process (here network time protocol daemon):

Info for all ntpd traffic:

Info for one of the ntpd servers:

You can also filter for hostnames, ports or IP numbers.

A single license is ~30 €/$30. Black Friday offer is much cheaper - coupons are available sometimes.
No affiliation with Objective Development here.