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ASP.NET Core
ASP.NET Core is a cross-platform .NET framework for building modern cloud-based web applications on Windows, Mac, or Linux.
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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.
It kinda hard as if you been used firebase mean you know functionality offline first. You can used any back end language like javascript(i not like it thou) or mostly i used asp.net or php(fastest for me). Storage is a bit odd , some people prefer to use directory and some use database.
You can check out https://appwrite.io/ - it's like firebase, but self-hosted.