One option is to turn on iCloud in the Photos Settings, and also check "Optimize Storage." Then you will get an exact copy of all your pictures in the "Cloud," and you can see them on the web at iCloud.com. The "Optimize" choice means that smaller copies of the pictures will be stored on your Mac, so they look just as good, but don't take up so much space. When you add new pictures to Photos, they will be sent to iCloud automatically. If you get another Mac, you can make the library in the new one the "System Library" in settings, and it will fill with all your pictures (over a few days.) Then the old library will no longer work with iCloud. 5GB of storage on iCloud is free. 200GB costs $10/month.
But iCloud is a synchronization service, not a backup. Since iCloud and your Mac are alway the same, if you delete a picture from one, it deletes from the other.
You can backup the pictures by yourself very easily (and ought to do that from time to time.) The best thing is to get an external drive-- even a flash drive would do for backup. (Flash drives are pretty slow, so they're not so good to actually run Photos from.) You can get good ones from Amazon, for instance. Faster is better. 1000 MB/s costs maybe $80, 80 MB/s is maybe $30.
You plug in the drive in to your USB port, and then run the app Disk Utility to format the drive as either APFS format or Mac OS Extended (Journaled) format.
Now, your pictures are stored in a Photos Library, usually in the Pictures folder. The default name is Photos Library.photoslibrary (clever). You just drag that over to your new backup drive. So now there's a copy on the computer and a copy on the external drive.
If your backup drive is fast, then you can double click on the library there, and Photos will open it and show you your pictures. It will all work like usual. You can even make it your "System Library," and you can delete the library that's on your computer.
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