Using homebrew, it is possible to back up and restore Switch saves. This can be used to transfer saves between emuMMC and sysMMC, allow you to backup saves for games that don't support cloud saving, or use saves found online (e.g. to unlock all characters in Smash Ultimate).
JKSV is the recommended save manager for Switch. It supports backing up and restoring user saves as well as device saves, which are used by games such as Animal Crossing: New Horizons. Instructions for use can be found on its GitHub README.
Checkpoint is also a save manager, but as it doesn't support device saves it is unable to backup some game data. While it does support cheats, which JKSV doesn't, Edizon-SE is recommended for cheats instead as it provides more features then Checkpoint. See Switch:Cheats for more details.