so i know most 3ds game save is stored on the cartridge itself. my question is if i sell the cartridge will someone else have access to my game save? or if i buy a cartridge from someone will i see any game saves on it?

Yes, that's the case

I bought some used zelda/pokemon games for the 3ds and some of them had savegames of the previous owner on them.

You can delete the savegames though, so it shouldn't be a problem

can i load their game save or no?

Yes you can

oh so the game save is not user protected?

Doesn't seem like it. At least that's what I've experienced with zelda/pokemon games

i see. and how do i back up my game save from cartridge? they can be moved to sd card?

I have no idea if/how that can be done. Never had a need for it

/e: maybe check this out
https://www.nintendo.co.uk/Support/Nintendo-3DS-2DS/Usage-/Save-Data-Transfer-Tool/How-to-use-the-Save-Data-Transfer-Tool/How-to-use-the-Save-Data-Transfer-Tool-740824.html

This post was edited by blah2k6 on Oct 25 2022 11:16am

Save files on ds carts stay with the carts
With a little tech and know how you can flash your carts to pc to back up your data, almost the same way you make a rom
I can't link anything for this so you will just need to research this on your own
Is there a particular game your wondering about? Because some games have options to save parts of data

fire emblem fates thanks