How can I find iPhone apps uploading a lot of data? Why do the iPhones in my home periodically use up all the upload bandwidth? Happens at least once a week, likely once a day or something.

I only notice when I get annoyed by slow internet.

How do I analyse which app is causing this this?
It is unclear whether Bandwidth is from Cellular data or Wi-Fi setup separately. So I will do both.

In Settings → Cellular/ Mobile Data, after waiting for sometime, all the apps that consume data appear in descending order of used data. If Low-Power mode is off and sufficient battery power is available, then Photos and videos and iCloud Drive files will upload to iCloud if the feature Use Data for iCloud is turned on. To avoid extra usage this should be turned off.

For Wi-Fi, such usage chart is not available in iPhone natively. Two indirect approaches are:


See the biggest files or app-usage of iCloud. Go to Settings → Apple ID → iCloud → Manage Storage. 
It is less accurate but go to Settings → Battery → Usage pattern. This can be a mix of a lot of activities but most apps that are at the top use a lot of bandwidth in my iPhone.