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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.
I created a GitHub repo for both backend AND frontend, which has the same project but in various different languages and frameworks! Frontend and backend are obviously different
I am considering adding Svelte to the frontend repo and a GoLang backend.
If you want to create mobile apps, you will need to create the app for both iOS and Android, that is two separate projects but there are frameworks that you can use so you only write the code once, I suggest Flutter.