Books by Jeremy Silman are famous for addressing the problems of assessing where you are in the middle game. The Amateur's Mind is a famous one. He attempts to explain where he sees amateurs going wrong and what they should actually be focusing on.
Fortunately, he did many lessons for Chess.com. They are still available.
https://www.chess.com/lessons/now-what
These lessons are not 20 minute lectures. They are a series of positions you have to solve, with hints.
I was watching a GothamChess video and he said that your plans in the middle game extend from the opening.
Where do I learn this? I have a copy of The Ideas Behind the Chess Openings by Ruben Fine, but I find it extremly dense, boring, and not fun to use. The last thing I want to feel like is being back in a lecture during 4th period high school.
For example, let me say that if I am White and my opponent and I are playing a book line and I come to the point where the line ends for both of us, how do I know which piece to move next? Do I attack on the king/queenside? Is there a piece that I should avoid trading because it is a key in the next few moves of the position?
Where are other resources for me to find and learn what the objective of a specific opening is? I have a diamond membership on Chess.com so if you know of anything in their list of stuff, I can access it.