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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.
Install code-server (https://github.com/cdr/code-server) on a server, and using it in your phone's browser to develop small web apps (works well in landscape mode with 6 -6.7 inch phones)
If you have a powerful processor and adequate RAM - you can do almost any thing that you do on desktop/pc
I use my iPhone 7 for a wide variety of creative video and audio projects. It's getting harder now though, because UI funnels that arrive with updates sometimes strip custom workflows I come up with. App developers seem to be moving to in-app purchases of preset packs instead of creative controls.
Here is a gallery of ~1 minute abstract videos with custom soundtracks. The apps I used were LumaFusion, Synthscaper, Fieldscaper, Defekt, FilterStack, Fragment, Kino Glitch, 8mm, Hyperspektiv, Plotagraph, and Virtual ANS.
https://lucidbeaming.com/glitch/
Here is a music video for U.K. metal band DAMIM that was made entirely on my phone.
https://www.youtube.com/watch?v=J6OZVXYamOQ
I have also used the phone as a movement based MIDI controller with AC Sabre and arpeggiator with Fugue Machine.
An old code project was to make a phone based web app to control Zynaddsubfx (synthesizer) running on a Raspberry Pi. Fun learning project for Node.
https://github.com/lucidbeaming/ZynAddSubFX-WebApp