Rom hacks really only work with a computer because you need a game file (ripped from a cartridge if you dont plan on breaking the law which 100% requires a computer) plus a patch file which then needs to be fused with the game file, it is really only possible on a computer.

Its possible to create one a phone with all the same steps but its really tedious and not recommended (especialy ripping cartridges or disks, youd need your own adapter for that its a whole thing).

if your searching for where to find the patch file's game banana and gbatemp are good sites to find patch files (note patch files aren't the full game they are just the rom hack itself, it still needs to be fussed to an actual copy of the game. )

(note to the mods this convention dose not break rule 5 as everything iv discussed has been related to legally creating a rom hack, and I didn't help this person with with piracy in any way, everything iv shared is related to the legal way to create rom hacks.)