I'm trying to explore the Vuforia API, but the documentation has only explanations of what the methods/classes do. If I want to learn how can I scan objects myself and add them to a dataset for example, what should I know first? And are there any simple tutorials to start with (Android preferably, I don't want to work with Unity).
What you are asking is not possible with Vuforia - there is no published SDK for doing what the Object Scanner does (and the Object Scanner itself is not even available for other platforms but Android), so you cannot make one of your own.
The only currently available way to use Vuforia with scanned objects is to use their app, and feed the scanned OD file into the Target Manager