I notice that in some games that your information is saved. e. g. Matchy Match, Pciomon

How do you do that?

DSET (and DGET) let you write and read a very small amount of permanent data.

Hey @PixelBytes, this is a good resource.. includes some things not covered in the manual: https://neko250.github.io/pico8-api/

Another good thing to do is to check out carts that use save data and then search for "dset" and "dget" in their code to see how they accomplish it.

See also: http://pico-8.wikia.com/wiki/Cartdata