I would like to know and understand the steps of SharePoint itself when it comes to uploading documents into a library (i. e. used as document management system) - also regarding the version control. Which system components are taking part here and in which order? I think any object model - especially for a document library or the version control of SharePoint would help me a lot.
There are three or four "object models" for SharePoint. They vary by where you're accessing SharePoint from, how you want to access SharePoint, and what language you want to use.
Since you reference 2013, Start here:
and then under "References" choose the particular library you're looking for:
- REST API reference for SharePoint
- .NET server API reference for SharePoint
- .NET client API reference for SharePoint
Look for documentation around lists/libraries... version control would be part of that, although it's a list configuration and you would need to specifically access the version history to get that type of information.