Hello! This is an excellent question, but a bit difficult to answer without knowing a bit about your process currently.
It is important to start with a line of action to map out the pose, and use volumes to note the positions in 3D space of the ribcage and pelvis, noting which directions they twist. I would recommend practicing using volumes, and learning perspective if you want to fully understand how things work in a 3D space. I will linik a few resources that will help explain it better than I can in text!
This channel has some very helpful videos! I recommend starting with this video:
For muscles, I recommend studying anatomy as much as you can through anatomy books, free online resources, or courses like this one by Ariel Olivietti that I always recommend! I really admire TB Choi's work as well, and how she shows her in-depth knowledge of anatomy. Her approach to figure drawing and drawing muscles can be seen here: