App that tells me what app is using data (Big Sur)
Activity Monitor gives a lot of info, including:

If you have the battery icon in your menu bar, you can see this:

Now, that says energy rather than I/O, but what's interesting is that it always says Photos.app even though the app is not running.  My guess is that even though it is supposedly not running, it is maintaining the sync between local and iCloud, i.e., network.
In an admin account in Terminal, you can use tcpdump to see details on what packets are being sent and where to/from
About a decade ago, my son used to play a game called RuneScape until I discovered that the Mac he used was exchanging huge amounts of data with the RuneScape servers when no one was logged in on that computer!  (And so I blocked all five servers.)