Nintendo Switch: Where does your saved data get stored?

Does it get stored in the Nintendo Switch console or in the game cartridge? Does it work like PS4 discs where when you save your game, it saves on your hard drive and not on the disc itself? If I were to buy a Nintendo Switch cartridge of the same exact game and use it, I would still have my saved data which is stored on my console and be able to play it on my new cartridge? I want to buy a new cartridge because mine rattles unless it's normal for cartridges to rattle? Is there a way to upload my saved data to online storage? If so, how can I do this?

Good question! From the support website:

On Nintendo Switch, game save data is stored on the console?s System Memory. This will not change whether downloadable software or software from a game card is being played.

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From what I know, transferring your save via System Settings will transfer all of your game data as long as you log in with the account the game is tied to.

I have tested using carts and digital games at the same time, and I can confirm that you can use the same save data on the console regardless if you play it on the cart or on digital, because the save data is stored on Switch's system memory instead of being tied to physical carts or downloaded games.

And if you want to back up the save data on online, you need to get Nintendo Switch Online subscription thing, then access Save Data Cloud Backup via Data Management setting. Please note that some games like Splatoon 2 wouldn't support it, though.