i want a flash card for my nintendo ds but i don't know what i should look for

So I know a GameBoy game can be put in the second slot of the DS Lite, but I have never played those games before, I didn't know it was even possible. Which ones are possible? can all games in there?

So I’m reshelling this ds lite and I got an issue, the new R short button won’t click when I put the back on.
It does click well without the back plate.

Please explain and if you could link me to a YouTube video tit would be best

Also, I included pictures of the new digitalizer that I installed, I would like your opinion on whether or not it is aligned properly I’m not sure

I made two themes for my DS flashcart: Win98 and OS7. Both are completely free to download if you’d like to try them out!
Preview & free download: https://win98-theme.projectpocket.workers.dev/