Data Hoarder

Many storage systems have features called "data scrubbing" https://en.wikipedia.org/wiki/Data_scrubbing , which Synology discusses here: https://blog.synology.com/how-data-scrubbing-protects-against-data-corruption

This will correct errors with drives, and potentially give some early warning that a drive may fail. You will also want to run SMART tests on your drives. Quick tests often (I do daily), extended tests occasionally (I do monthly).

The backup software should also have a way to verify the accuracy of the data, and check that the data can be restored. On Synology, HyperBackup has backup integrity check https://kb.synology.com/en-us/DSM/tutorial/What_is_backup_integrity_check_for_Hyper_Backup_tasks .

if you use real backups, and not just simple copies, then your backup software has verify function. For simple copies you should use hash files or something that can build a hash database and verify it. Btw. you should already use hash checking for live data anyway. For archiving you can create winrar archives with 10% recovery record, so it can self-verify and self-repair easily.

Ideally you would have generated and saved a HASH before you copied your files as a control. Otherwise, it's just a probability game. If the HASH on copy 1&2 match, but doesn't match 3, then the probability is 1&2 are correct. If all three don't match, you toss a coin.

If you're on Windows, I recommend using Teracopy for all your file copying (always copy, never move!) and set verify on, which will perform a CRC and generate a HASH which you can then save. You can also use it to Test your files after the fact and generate a HASH.

I use snapraid as one of my backup methods. Mainly for long term mostly static archive backups. Things that no longer change, but is added to, and I still want to have accessible read-only. Not for daily backups or for frequently changing files or folders, nor for "permanent" off-line cold storage.

https://www.snapraid.it/

I use 8 storage drives and two snapraid parity drives.

Using snapraid I can then easily verify that all backed up files are 100% OK, exactly as they were when I had just backed them up.

Snapraid can detect and fix bitrot (has never happened so far), undelete accidentally deleted files or folders and even recreate up to two failed drives.

When I backup/archive files, I simply copy them to one of the storage drives and then ask snapraid to update the parity.

Done!

how do you make sure that EVERY FILE in each copy is good.

Checksums.

Personally, I use TeraCopy to safely copy a folder/file from my main drive to my backups (there’s even an option on there that will save a checksum of said folder/file on the backup so that I can later run that checksum and see if anything has corrupted).

What do I do if there’s corruption? Simply delete the corrupted files and replace them with good copies from other backups.

This why you check your backups periodically and replace the bad ones with good copies. If you're asking how you know what's good and bad - traditionally and fundamentally, even if many people here dismiss it, the storage already has checksums, that sneaky bitrot when the storage will give you slightly altered data (instead of saying "Error") are so small that most people would never encounter this. Now of course serious data hoarders would use checksumming file systems, will do extra checksums for any archived data, also all archiving formats or backup formats have their own checksums too, if one would use that instead of dropping the files in the regular file system.

Versioning
My backups are incremental (Mac TimeMachine and Arq)
If I find a file is corrupted, I can restore an earlier version

It's something I need to take a look into more if I'm honest, I checked all my media in VLC looking for duration and replaced any with no duration that wouldn't play.

I've got an old backup I can refer to and when I sync to my Synology the deleted items don't get removed so if something gets removed by mistake I have a couple of places to refer back to.

There's still the risk of ongoing corruption/bit rot so I installed Checkrr last weekend to try to flag problematic files.

Take a look: Checkrr