Can I use the same save file for different copies of a Switch game?

Yes

From Nintendo directly:

Where is game save data Stored?

• 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.

• Game save data cannot be saved or copied to a microSD card.

Can save data be used between physical and digital games on Nintendo Switch?

• Save data is stored in the System Memory, and for the most part, it is associated to the individual user. So long as the same user is selected on the console, the save data will not be impacted whether the software is played from a game card or downloaded from the Nintendo eShop.

Yes. Switch cartridges are read-only, your saves are stored on the system.