[diem_perdidi]

I'd recommend starting with the first blogpost of this series (https://acoup.blog/2022/09/16/collections-teaching-paradox-c...) - it's a good read! It delves into the differences with other Paradox games - the second paragraph is actually a short answer to your question:

  This first part is going to focus on the way that Crusader Kings understands rule and rulers. This in particular is a fascinating place to start because unlike all of the other Paradox grand strategy titles, Crusader Kings III doesn’t actually feature any states in the narrow sense of the word; none of these rulers have a monopoly on the legitimate use of force. This is an enormous difference between CK3 and its sibling games and well worth diving into.

[deaddodo]

I would recommend the other two series as well. First EU4 (https://acoup.blog/2021/04/30/collections-teaching-paradox-e...) and then Victoria 2 (https://acoup.blog/2021/08/13/collections-teaching-paradox-v...).