OPDS (Open Publication Distribution System) is protocol in the background. Jellyfin has a plugin for OPDS and works by simply dropping a file in specified directory, but it doesn't support multiple catalogues (i.e. one per library/directory).

Protocol is atom based, chatgpt was able to make a custom OPDS server for my needs within minutes, it took another hour or two to fix and customize generated code.

Are you able to sync progress? I can access my calibre-web library, but I've been unable to figure out how to sync progress.

To sync progress, you can use `https://sync.koreader.rocks/` as a custom sync server. While in a book, top bar, settings, "Progress sync".

I think it works using the file name of the epub. Not sure.