Evaluation of Key Squares

Hello my favorite Chess Community,

I am struggling right now with a specific topic and would really like to understand the evaluation process in order to determine it. I am trying to understand the concept of Key squares in Chess and I hoped you could either explain the specific thinking process (just for the evaluation of all the key squares in a specific position) or at least point me to some "really" good resources that do explain it step by step to me so even an idiot would theoretically be able to grasp it. Thank you very much for all of your help on this topic in advance. Below is a specific position that is included in one of my study resources. Here the following marked with a blue circle are "apparently" the key squares. e2, e3, and d4 are Key squares in the below diagram. I wonder why? 

Example 2 is the following... where the key squares are marked in green.....

Here is example 3 

If you could kindly try to explain the methods in order to evaluate these specific key squares in those diagrams and / or recommend me 1 or 2 books which explain this in a step by step approach, that would be very helpful. One more time, thanks a lot. 

They concept of key squares isn't something I usually think of. The usually calculate a bit from understanding of king opposition and try to solve how to get where I need to while shouldering their king, or similar. I guess that's key squares in different ways, but I usually just identify one or two locations through reasoning.

This may sound simple, but lichess has a really good "lessons" series on the pawn key squares for different situations; you may go through those if you want to really reinforce the concept.

Yes, this makes sense, as the other side will have to give way after reaching those squares but what I don't fully understand at the moment is what is the exact method to figure out that these are critical squares? I do know about the concept of Opposition but I struggle to determine right now why for example in example 1 e3 and e2 are critical squares? I can see that d4 is a critical square as whoever side's King reaches this square will either promote the King or be able to defend it as the Key squares are laying just in front of it being c5,d5 and e5. In example 2 I believe I have understood why these specific squares are "critical" squares... it's because if the white King can reach any of them the black pawn will fall. Either by reaching e2 or first of all f2 and the idea of Opposition comes into play and I would be able to move the d pawn to d4 and then infiltrate with my King from behind taking the black pawn and win the endgame! Example 3 I also still don't fully understand. Yes, I mean it makes sense that the white King tries to take the pawn on g4 in order to create a passed pawn. Okay, I can probably get my head around this example too come to think of it. I was just wondering why c7 for example is not also a critical square, as white's king could infiltrate and eventually try to queen his d pawn. After counting I can see it would take white a total of 12 moves to queen and it would take black a total of 11 moves to queen. Is this the reason? I think I'll take a look at those lichess lessons that you suggested.