I found that the best way for me to internalize a particular subject in hard science is to follow its history by reading the original papers. It's a hard way to do it, but it helps you understand the motivation, the progression of ideas, and the rigor that went into it. Anything else doesn't quite work for me - I feel like I "sort of" understand, but five minutes after I close the textbook (or the YouTube video) the…
..but reading the original papers? I can't imagine ever deciphering Newton or Maxwell (before Heaviside fixed up the equations, it was some horrific mess of like, 20 equations with 20 unknowns). There are great benefits in a clean, logical development.
On the other hand, I hear Dirac's papers were remarkably clear. Maybe reading the originals is easier for math?
"..if one wishes to make progress in mathematics, one should study the masters and not the pupils" -Abel