"The absorb what is useful (to you) - how will you know what is useful & where to get it?"
The only way that I know of is through what has been called "Alive Training". Unless you are working against a resisting opponent, then there is no way that you can honestly tell what problems you will have to face, that you don't already have an answer for. Our instructors can talk and talk and talk, but until we get taken down and tapped out, or blasted across a room, we will never understand.
As far as to where to get it from...well, simple, find people that have answered that problem before. Years ago I was on the opinion that I didn't need any grappling training. I figured I wrestled for four years (20 years ago LOL) and what more did I need. It was only after a student of mine took me down and tapped me out over and over again, that in finally sunk in. I began training with Matt Thornton and learned many lessons.
I sought out my training. How I knew who to seek out? Well, Alive training told me what I needed to work on.
I have a JKD instructor (Steve Golden), but when it came to ground fighting, he and I didn't bullshit each other into saying "What I have will work, I just have to learn some super secret whatever..." You know? I called Matt.
Now, I was fortunate enough to have an instructor without an ego. He suggested I talk to Matt, but he didn't care who I trained with, as long as I did to solve my problem.
Anyway, sorry for going on so long about this. LOL