HTMX, like it's name implies, is focused on enhancing HTML. It allows HTML attributes to make AJAX requests and update the HTML on the page with the response.
It's not tied to any server side framework making it usable with any of them.
Most of the functionality is designed to be done on the client site. For example what element to update, what element to update from (if needed). However, the server can override some things using HTTP headers.
It has support for managing browser history, morphing, transitions, validations, and debugging. Websockets and server sent events (SSE) are supported via extensions.
I like the overall simplicity of HTMX and can see why it's gaining so much popularity.