Backing up a phone, specifically
Phones hold the things people are least able to replace and are most often backed up incompletely.
Photographs and messages are the irreplaceable items, and they are handled differently. Photo backup is usually automatic once enabled; message backup frequently is not, and varies by platform and by messaging application.
Check what the backup actually contains. Several platforms exclude messages from certain applications, or back them up only to a separate service that has to be switched on separately.
Free storage tiers are commonly too small for a modern phone's photo library, so backup quietly stops. The warning is easy to miss, and the usual discovery is after a device is lost.
Test it by restoring something. Checking that a deleted photo can be recovered takes a few minutes and is the only way to know the arrangement works before it is needed.
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