How work the banglejs 2 and what are our possibilities with it?

Is the firmware open too? Can we look at it / participate in it's evolution?

Yes, it is built from. I have never built the whole thing, but it is possible.
It is written in C and supports the whole Espruino based product range.

https://github.com/hughbarney/Espruino

Not sure where is defined that the press of the button call for the launcher through

This is done by each App using the
https://banglejs.com/reference#l_Bangle_setUI

It is possible to build a clock app that wont restart the launcher. If you set that clock as the default clock the only way out of that situation is to delete the app using the app launcher or the specific files through the IDE.

The behaviour of the launcher is controlled by the specific launcher app. You can choose from several different launchers.