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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
The source of the app is freely available under the MPLv2. I look forward to your contributions. https://github.com/jellyfin/jellyfin-expo
If you mean for music, the music-specific client finamp has offline playback/downloads on both iOS and android. Highly recommended.
For #6, TMDb Box Set plugin should get you started but I don't think it has the ability to set criteria for making collections (like must be in 4k). My JF instance is down right now so I can't check :/