Time Capsule back up
What is the best compatible back up for my iMac 21.5 ?
iMac 21.5″, macOS 10.15
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If you are asking about Time Machine or Time Capsule?
Time Machine is an app on your Mac:
Time Capsule is a device you need to purchase:
The Time Capsule was discontinued a number of years ago, so while it was very convenient it can no longer be considered "best". In lieu of it, or in addition to it, you can use any generic hard disk drive: Backup disks you can use with Time Machine - Apple Support. Get one with sufficient capacity, which is a function of how much storage you actually need to back up. As a practical minimum the backup device needs a capacity of at least twice the amount of storage you actually use. Three times as much is even better. Hard disk drives can be purchased in 5 or 6 TB capacities for about $100.
"Best" is also a backup device that you will actually have and will use. In that sense it is best to have multiple, redundant backup drives. That way, when one of them fails you can just throw it out, and never be without at least one backup.
To learn how to use Time Machine please read Back up your files with Time Machine on Mac - Apple Support.