Topic: If I have a digital copy of a game which has been transferred to my switch, will a physical copy override the saves on it?

This is kind of a roundabout situation that I am in, but my partner has a copy of fire emblem on she switch before we moved in together and transferred it over to mine when she had to leave her switch behind. The biggest problem with this is that I want to play the game, but since it was a digital game transferred through her Nintendo account, she can only play it on her profile and I don’t want to override her save file. I was recently gifted a physical copy of the game, which I was super excited about, but I am now worried that the same thing will occur with the physical copy of the game and erase her save file since it wasn’t initially from this switch. If I insert and play the physical copy from my account will her account will have all the data from before?

If you use a physical copy, anyone will be able to play it and the saves will not be overwritten as therly are attatched to the user's nintendo account. No need to worry!

With only a few exceptions (most notably Animal Crossing) save files are specific to accounts rather than the console.

In the case of Fire Emblem, you will not overwrite her save file if you play the game on a different account.

In terms of saves digital and physical behave the same. The difference is in how the games recognise the licence. A physical game has to physically be in the cartridge slot to play, if it's in the slot any account on that console can play the game. Digital games behave the same for the primary console, the primary console effectively behaves as if the game is permanently in the cartridge slot. But additionally digital games allow you to play on another console with your account by checking in and confirming the primary console isn't currently playing. A feature which is basically the digital equivalent of walking over to the other console and taking the cartridge out of the slot

For saves the behaviour is the same for both. If you play the game on your account it will create a save on that console for that account. Generally. If you have NSO that save will be synced to Nintendo's servers so you can then play on that same save on another console with that account. Generally. If you are playing with your own account on your own console there is no way to overwrite someone else's save

The exceptions above? As @Matt_Barber said, Some games don't have one save per account, they have one save per console. Notably Animal Crossing but also some "party game" titles from memory. Basically games where it makes sense for progress/play to be "shared". Also there are some games that don't allow cloud saves, usually titles where cloud saves could potentially be abused to gain a competitive edge (Pokemon, Splatoon). But also all of the games where the save is tied to the console not the account

It's worth noting that unlike Nintendo's older consoles, Switch game save files are always stored on the console, not on the cartridge. So if you play the game from the cartridge instead of digitally it'll just use the save file that's already on the console.