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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.
Obsidian[1] is an all-in-one solution that doesn't have much in the way of lock in. It fundamentally runs on Markdown files sitting in your filesystem (and you can even "reveal in Finder" from the UI). It has its own custom indexing to make things quick. They offer sync as a service, but people also use other sync solutions. They have a mobile app version to access notes elsewhere.
Beyond just folders for categories and tags, Obsidian also supports easy linking between notes (and shows backlinks for the current note).
In the end, it's great: should I decide Obsidian is no longer right for me, I have a bunch of Markdown on disk that I can take elsewhere.
[1]: https://obsidian.md/