iCloud & iCloud Photos · iCloud storage space and plans
How do I see which apps and backups use my iCloud storage?
Short answer: System Settings → your name → iCloud → Manage lists everything using your iCloud storage, largest first — photos, backups, iCloud Drive, Messages, Mail and app data. You can delete backups and app data there, which removes them from every device.
Open System Settings, click your name, then iCloud, and click Manage (or Manage Account Storage). You get a list of everything using your iCloud space — Photos, device backups, iCloud Drive, Messages, Mail and each app's data — largest first.
From there you can open an item to see more and, for backups and app data, delete what you don't need. Deleting removes it from iCloud and from every device that uses it, so check before you remove anything.
To see how much space iCloud files take on your Mac instead, see how much space iCloud uses on my Mac.