Answer 1

Hello, Szymon Kopeć !
For android I write on Java because Google has long loved this language, and made its mobile OS (android) in this language. Under the hood there is a special Linux kernel and a reworked virtual machine, which serves as a layer, between attachments and the kernel. Those. almost any application is run in this virtual machine, so it should be written in Java (if more precisely, the virtual machine starts the bytecode that is obtained after compiling the programs)
https://www.sololearn.com/Course/Java/?ref=app
https://www.sololearn.com/learn/1439/?ref=app

Answer 2

You could consider learn Futter, a new Google tool that relatively more easy to learn and create apps compare to using java or swift language . And it produces cross platform native mobile apps which can be run on Android and Apple phones.

Answer 3

kotlin will also be good

Answer 4

better take a php course