Can't backup game save on switch, then what am I doing with these memory cards with switch games data on them?

Yeah.

The website [0] is pretty clear that the content of the game can go into a SD card, but the game save resides only in the internal memory.

You can find some ways to get them with some modding but nothing official.

[0] https://www.nintendo.com/ph/support/switch/data_management/i...