Epub/cbz/pdf and Reader

I’ve installed (in turn) epub/cbz/pdf and Reader. How does one use the apps? If I select an epub file, the file is downloaded instead of opened.

this one you seem to refer to is only supported until NC 23…

In case of a newer NC version, it is perhaps just not enabled properly. The only one from the store that seems to be compatible with NC 28 is this one:
https://apps.nextcloud.com/apps/epubviewer

In theory, you are right, after enabling it, you just click the file in the file view and it should show the epub in the viewer.