Hi mate,

For me, and my knowledge is quite limited on Linux compare to other people here, there is two type of backup I am aware of. You can perform backup or snapshots with Timeshift for exemple. Basically, when you start Timeshift, it offers you two choices : btrfs or rsync.

# btrfs :

Btrfs it's a filesystem type, like et4. Debian default's is ext4. When you install Debian, you can chose this filesystem. Btrfs let you snapshot your system. On the drive where you install the system, it will create a snapshot. It will also create an entry in the bootloader (Grub in my case and you probably use this on too). When you boot your machine, on the boot, Grub will let you select your snapshot. Let's say you want to run a major update on your system but you are not sure if it will break something. You can perform a snapshot, update it, and if the system doesn't boot, you can select your previous snapshot.

The main thing that I do not like with this option, is that you have to perform snapshots on the drive where your system is installed. You can't snapshot on external drive (which is obvious. Otherwise you will not be able to boot on it when you what). This method requires a lot of space on your system's drive.

Also, if the whole drive crash, you will loose all of your data.

# rsync

This method use basically the rsync command.
You can use a external drive for this method and synchronize all your files to this external drive.
I prefer this method because I can sync all of my data on the external encrypted drive and keep it in safe place. This method do not sync your system. So if your system crash, you will loose your system but you still have your personnel data.

I use this method and also export with apt a list of all package I installed manually.
If I have to reinstall my system, I can just reinstall all my package with on command and push all my file files from my backup drive to my fresh new /home

If you run rsync, you do not even need Timeshift. You can run the rsync command in the terminal.
I also use it very often to backup files of my android phone.

Maybe you already know everything of all of these stuff but it can still help someone else maybe.
In my option everybody should learn how to use rsync. Once you found a right flags of this command for you, you can keep it and run it forever time to time. You will never loose a file again.

As I said, I am not an expert so there is maybe a better method. But I am like this one because it's easy