lemmy.ml runs on the vanilla Lemmy codebase, so from a software development standpoint, you mostly can’t pick & choose which instances you’re helping. You can work on whichever features/GitHub tickets you’d like to see on lemmy.ml (though keep in mind that a few features are optional/configurable, and the lemmy.ml admins and/or user base might not want some enabled. It’s a good idea to read the room before investing the effort).