Too much to say, and there is no simple answer, but I can certainly tell you one thing: Drawing from life, the point of it is not to achieve photo realism. You can go for that if you like, but it takes time, patience and skill. No, the point to drawing from life is so you can study form, shape, line, space, value, proportion, and if you're using a model, you need to see them in real life because real life is the only way to accurately study the muscles, and how they look beneath the skin. Meaning, essentially, what effect do the muscles have on the skin when they are flexing, extending and morphing into different shapes with each movement.
This provides a good, solid, basic foundation for artists. Artists who wish to draw realistically, or even just representationally, need a strong grasp of the basics before they can advance their own skill, or help others attain a greater level of skill themselves.
Doctors heal you, Artists immortalize you.
"Those who can, do. Those who can't, teach" - bullshit.
The usual staples for anatomy: