How to create an electronic library application for a university on a smartphone?

Question

Electronic books

At the moment, I tend to write in C-harp in Visual Studio, because there is cross-platform and the language is more understandable to me. The initial interface was developed in Android studio in Java, but due to poor knowledge of the language, difficulties arose with connecting the database.
And what services should be considered at the moment for creating a database, and its subsequent integration into the application?