No prob, happy to help!

This might be a silly question, but when you backup, do you need extra storage space 1:1 with whatever you're copying? Or does it compress the files somehow to keep those backups in a smaller form?

Not silly at all. I was actually going to mention it, but thought the comment was getting too long (lol). It depends on the backup being used, but Kopia and restic both use compression and store the data in archives.

I think, depending on the compression algorithm used, you can save a fair amount of space, but it does add up over time if you're backing up new (large) files often.

The application I use (BorgBackup, sadly not on Windows) is very similar to Kopia and restic, and just as an example, I ran a backup of brand new files that added up to around 14 GB on my computer, but the archive was only about 10 or 11 GB. Might have some numbers a bit wrong (not at my PC right now), but it's somewhere in that ballpark in terms of disk space saved.

They also won't backup files twice unless the file has changed, so large files like videos should only be copied over once since they rarely ever change unless someone changes the metadata or something, which most people won't do.

The only real downside is that you won't be able to just browse the files like normal and copy them back over. You need to first open the archive with the application, but then you can pick and choose which files you want to restore.