Mobile applications and how do they interact with the server?

Question

Mobile development

Hello.
My development in development has come to the very moment when it is worth choosing a specialization.
I like the backend, there is still a lot to learn there. And before starting the in-depth study, I analyzed several articles about the development trends of programming in general. People are switching to mobile applications, few people use simple sites, etc. I learned that, for example, YouTube is written in python, and applications for mobile phones are written in swift or java ... In short:
1. Can you write a GUI in java and swift?
2. How is the interaction between the mobile application and the server? by REST? 
3. Why not try to write an API that the PHP application will use? is this possible?
Why I ask questions: I would like to choose some programming language for further development. Now I am writing in NodeJS. I want to work with a different JS syntax on the front and get bored with the back, dig into other languages, see how they work.

Answer 1

In cases of "especially abnormal programming" you can communicate only through XMLHttpRequest,
and allow cross-domain requests allowed in php.
I think it's all about the required level of security.
For example, I'm not going to write software for commerce...
This level of complexity is enough for me to start with.
For example, a toaster can be done like this - don’t care that someone hacks someone - well, it will start a new page and restore data from backup ...