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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.
They offer Auth, but their auth product is basically a wrapper around GoTrue + a private auth schema.
Similar with object storage. They rolled their own, but it's basically a small SDK and they have a hosted CDN, but all the data about your objects is in a storage schema.
There's nothing wrong with this and they're pretty open about it. But the SDK they provide for direct database operations is the weakest of the ones I've used, when it should be the strongest I think. It leverages PostgREST which is a tool for auto generating REST APIs from schemas. From the README: "The goal of this library is to make an "ORM-like" restful interface."