I simply do not understand how medieval politics worked because I do not understand a) who had power over whom or b) how individuals justified their behavior towards one another.

Prior to the Protestant Reformation Europe was Catholic. And it seems like the Pope has power. The Pope controlled something called the Papal States (although I have no idea what this is), three Archbishops were electors of the Holy Roman Emperor, and the Pope was able to nullify oaths sworn to the Holy Roman Emperor during the Canossa affair which itself resulted in a rebellion against the Emperor. So clearly there was power there.

Yet wars between Catholic monarchs (or claimants) happened. Did the Church support one side or the other? What happened if the wrong side won as it were? Or when the Pope was specifically on one side (e.g. during the Italian Wars)?

For that matter, how much power did Kings, Dukes, Emperors, etc. have over their noble subjects? The Holy Roman Empire had a fair number of wars where its member states were participants on opposing sides. The King of England used to rule parts of France (Brittany, I believe). How did people know what their rights and obligations to different groups were?

And how were even further foreign relations conducted? How could another state do diplomacy with the Holy Roman Empire (or any composite monarchy) and how were Christian states able to do diplomacy with non-Christian states?

It seems to me that the modern view of politics is much simpler. The actors are states, states possess certain rights over their citizens/subjects and territories, which they describe in some way or another, and they join different international institutions which have limited ability to enforce their rulings.

But I do not see the logic in medieval politics. I am not sure who the actor is (the individual royal, the various realms ruled in personal union, the different estates within each realm, more transnational entities such as the Catholic Church, etc.) and I am not sure how they know what obligations they have and what powers they have.

To be clear, I assume that there is an internal logic to this. And I assume that it is one that changes over time. So I am asking what that logic is.