Confused about where the data saves?

So I recently bought a 2DS from Vinted, and although it seemed to run okay at first I’m getting a reoccurring issue of errors in my games. The console itself has been the common denominator so I’m buying one from CEX instead (and it’s pink yay!) I’m also wanting to get a new SD card with more storage.

The thing is, I’m not sure how much of my data is going to transfer. All I mainly did on the old 2DS was play a few hours of Pokemon Alpha Sapphire and make a couple miis. Does the game data save to the cartridge itself (I hope so)? What about my miis?