Hi, hope you're having a great week, well when you tackle anatomy, or anything really, it can be quite daunting, so we try to simplify stuff with simple volumes as a base to make thing easier on ourselves, the better you are in these simple overlapping volumes the easier other things will become.
Try to take things a bit further for a bit of extra challenge, make these volumes interact with eachother, no need to shade or anything, just make then be one inside the other and try to make the edges in the connections disappear, try to deform these volumes too, it's VERY challenging but if you want to improve challenge is the word, but know your limits tho, if it's too hard tone it down a bit, i did a quick example, hope you don't mind
Just so you know, you're doing great so far, keep at it