I have learned many of these things at some point. Unfortunately, I have also forgotten most of it. Some of it I still remember to some degree, or just the basic idea, and can probably reconstruct it given some time, or easily look it up and remember. However, many of the things I have completely forgotten, and it would take more time to understand it again.

I am not sure I can keep so many things active in my memory + also other things, depending on what I am working on in the last years. If my work in the last few years does not need some of this knowledge, I keep forgetting it.

Maybe my memory is just bad.

I think many of the things listed here are somewhat specific to what the author has worked on, so that it's easier for him to really know all this.

Maybe the point of this post is also more generic: Try to have an active memory over a diverse set of fields.