[Essential Phone Questions & Answers] DoR3M3:
Ok I have managed to make a Backup and then Restore it! ;)

MAKING THE BACKUP

These are the Steps I have taken, and take note, I do reboot, which some might assume isn't needed, but I have found, that if I didn't, then Magisk wasn't getting flashed in properly. Sometimes you just need to reboot between steps...

When you flash a Custom Rom, that should be on Slot A, then if you root with Magisk, Magisk will be flashed to Slot B. So now your BACKUP & RESTORE will be on Slot B! But some times I have seen people get their Slots flipped around, so however it ended up, the Slot you are making your BACKUP on, is the Slot that Magisk was flashed to, not the ROM! ;)

1. IMPORTANT - TWRP does not backup the Internal Storage so this is a simple way of backing it up! Advanced > File Manager > data - RENAME the media folder > media.bak.

2. Now you are ready to make a BACKUP! All you need checked are Boot > System > Data - Click on (Auto Generate) to rename your BACKUP!

3. When the backup completes, now go back to Advanced > File Manager > data and you will see media, which is the new folder TWRP created with your backup in, and your old one you backed up, media.bak. Now you need to rename them both! Rename the new one to media.new and rename media.bak back to it's original name media. Now here's that reboot part I was talking about before, now Reboot > Bootloader.

4. For this step I'm assuming you're Rooted and using a Custom ROM, if that is the case, when you reboot back into TWRP > Install > lower right Install Image and pick the boot image, and make sure you flash it to Boot. :)

5. Now go back and Install Magisk.

6. You have now successfully made your TWRP backup, now reboot the system.

7. Once booted up and back in your system, start up your File Manager, I personally use and like FX File Manager with Root Access. Now you'll navigate to data and you should still see your two directories media & media.new. In FX File Manager I have this path, data > media.new > 0 > TWRP > BACKUPS. You want to copy BACKUPS to your Internal Storage TWRP directory. So in your Internal Storage you end up with TWRP > BACKUPS. Once you are done copying it, it's safe to delete the media.new directory, but just make sure you have copied the BACKUPS directory.

8. To test that you have your BACKUPS in the right path, boot into TWRP and click on Restore, if it shows up, you placed it in the correct path!

DOING THE RESTORE

With the A/B partitions, if you are rooted and running Magisk, then you will have noticed, that you installed your ROM into Slot A, then you should of rebooted back to the Bootloader, rebooted back into TWRP and installed the boot.img and Magisk, on Slot B. But as I mentioned above, some times people get their Slots flipped around, so remember, whichever Slot your flashed Magisk to, is the Slot you RESTORE on!

1. Boot into TWRP, which I'm assuming by my above statement, you are on Slot B, click Restore, click your backup, then make sure all partitions are checked, and then swipe.

2. Now reboot to the Bootloader, then reboot back into TWRP and Install your boot.img and Magisk.

3. Next go to Advanced > File Manager > data and you'll now see media & media.bak, confused? LOL Just think about it for a second, and you can delete media.bak.

4. Now reboot into your system and enjoy! ;)