Do all my devices share one plan or domain need a plan for each?

iCloud storage is linked to the Apple ID that you used to sign in to iCloud on a device. All devices using the same Apple ID for iCloud are sharing the same storage plan.

FWIW: iCloud is a sync service. This means that e.g. photos, contacts, iMessages, calenders are synced between all your devices on which you are logged in with the same Apple ID.

It also means that if you delete e.g. a photo on one device it is deleted on your other devices too.

If you want to create backups: use TimeMachine and an external drive