The plugins in the expo/config-plugins repository were written by the Expo team because the modules they are for did not already have their own config plugins. Ideally each project would write the config plugin needed to use that module in an Expo app.
So if you want to see how to write your own config plugin you can look at the source code for the config plugins in that repository to see how they were written.
Also check the documentation if you haven’t already.
I have not yet written one myself, but what I would do is figure out which files you need to modify, then check the Config Plugin docs Mod Plugins section to see if there’s one that can modify the file you’re interested in. Then check the expo/config-plugins repository (or other projects with config plugins) for examples of how to use that mod plugin. Then if you get stuck with that, ask specific questions on the forums.
I hope that helps.
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