rsync, SyncThing, or Nextcloud. I use all three, often on the same machine at the same time, for various purposes.

rsync is great for scheduled syncs (but intentionally not TOO frequent – where blowing away one copy would almost instantly blow away the other copy could be a concern.) and outstanding for one-off syncs. With the right command line options it can even create a rough approximation of snapshots with copy-on-write rolling incremental backups. I believe this is the original documentation for that particular trick although it has spawned many genuine backup tools which operate very similarly using rsync under the hood, like rsnapshot.

SyncThing is great for large, intensive datasets that you want to keep perfectly synchronized across multiple computers at the same time. One example is my saved games, so without relying on Steam “cloud saves” or anything, I can just save my game and hop from gaming on one computer onto my laptop and pick up right where I left off.

Nextcloud is what I use for keeping separate copies of all my documents, photos from my phone, etc. It’s easier than SyncThing to setup and manage in my opinion, it’s simple and reliable and reasonably user-friendly in most use-cases, and seems to be more compatible with a lot of software due to its similar design to something like Dropbox. It isn’t ideal for handling huge numbers of files or very large folders in my personal opinion, but it does work if you want to try it.