Thank you so much for the detailed, step-by-step instructions. I have now also uploaded a build to the AppStore, with an in-app extension. The key step that I had missed in my previous attempts is number 3 on your list: remove the Info.plist for the Extension that was generated by Xcode (and probably number 4, delete the INFOPLIST_FILE in build Settings). The extension is not yet doing what I want it to, but that's another problem. I can upload, so it's worth pursuing this line of work.
Topic:
App & System Services
SubTopic:
General
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