What is my iPhone downloading in the background? Is there any way to find what apps are downloading stuff in the background? And what stuff they are downloading? I've done a little research and gather it's the Background Transfer Service doing the work. But no hint of how to monitor what its doing. This other question is a bit more general and the answer was "there's no way to know" but it's a bit old and the answers aren't authoritative.

About once a week when I plug my phone in and it's on WiFi I find it's suddenly downloading hundreds of megabytes of data. I can figure out a bit of what's going on from my router, an HTTP connection to aaplimg.com, but all that tells me is it's something on Apple's CDN. Short of running a packet sniffer I have no way to see what is being downloaded or by what.
Do you have iCloud Drive and/or iCloud backups enabled? If so, that's what it's probably doing. You may also want to check which apps you have enabled to refresh in the background. See: Settings > General > Background App Refresh