I was wondering wether there are any tutorials related to the “XML programming”?
I honestly don’t understand what it does and have so many question (e.g. how do i set a variable, can i perform mathematic calculations (sin/cos), …).
Try W3Schools Online Web Tutorials.
Haven’t used their XML tutorials, but a recent in-game panel project I’m working on has needed me to brush up on HTML and Javascript. W3Schools is now my ‘go to’ source for this kind of thing. Loads of excellent information, tutorials and it’s free 
XML is only a schema, it is not a programming language.
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I know, which is why i put it in parentheses.
But in the aircraft exterior, there is some sort of programming. E.g. when a simvar changes, you can call an event. Inclusing some if/els(e) statements. How does this “language” work?
I can’t really comment with any authority in this context, as I have not done any modelling in this environment (though I hope to get round to it at some point when I am less busy), but any logic or instructions will be actioned by whatever application is parsing the XML, so any syntax will be dictated by that underlying application.
Let your .xml’s follow you - not you they.
Just get your Folder Hierarchy set. Obey the Conventions for naming, etc.
Unless you’re doing highly complex aircraft and VC cockpits - which I’m not - there doesn’t seem to be much reason to go down the Rabbit Hole.
However, if one was so inclined to do so . . . then check this out;
https://docs.flightsimulator.com/html/Additional_Information/File_Formats/XML_Files.htm
And deeper;