How are cross-platform mobile applications created?

Question

Database

Tell me, please, how mobile applications are written in the case of cross-platform development (for example, xamarin):
1. In what languages ​​is the application interface written? (Do I need knowledge of html, CSS and JavaScript? Or is the entire interface written in XAML?)
2. What database management systems are used in mobile applications? (previously worked only in MySQL)
3. C# is used in xamarin. How hard is it to switch to it from php?
ps Probably, these are only the very first questions of the beginning of learning mobile development. If you have time, please describe the PRINCIPLES of mobile application development in more detail.

Answer 1

1) read the documentation for Xamarin
2) usually get by with sqlite or other embedded DBMS. The application is connected to the server via REST.
3) Depends on the level of knowledge of php and the ability to learn, find and understand abstractions, etc. In general, you should not drive at all, it's easier to learn.

Answer 2

1) The interface is written either in C#, or in XAML, or both are combined. You don't need JavaScript/HTML/CSS You
can see more details here: Xamarin

Answer 3

1. Xamarin GUI is created separately for each platform. The real cross-platform solution is to draw the interface via OpenGL (I'm using the Corona SDK).
2. SQLite
3. Much easier than learning with no programming experience at all.