Use Aldiko Next
Play Store link:
https://play.google.com/store/apps/d...aldiko.android
To connect to Calibre, start calibre content server as you usually would. In Aldiko Next, you'll find a "Catalogs" tab. In there, you add the server address of your calibre content server. Don't forget to add /opds after it. Mine, for example,is:
http://192.168.1.7:8080/opds
And if you connect your server and your phone to the same network, it should access your calibre books, which you can save and read in the app.
I love Aldiko because it respects the book's CSS, ie, it shows the book as the publisher intended. It is based on Readium-kotlin, which works closely with w3c and will generally have the best experience.
It isn't perfect, of course. Some issues I have with it are that it doesn't have an offline dictionary. Another is that it doesn't have the ability to zoom in on images in epub. However, I have reported this and they have responded that they are working on it. I previously have reported some issues which they fixed quickly, so I like the devs, too.