Really nice!

@Maria J Cool, let's see! 1. The plane change from side to front is actually shaped by the ribcage - it happens where the bone ends and the costal cartilage begins (the ab muscles just lay on top of the ribcage and follow its plane change). Remember the External Oblique is the top layer of the ab muscles and its aponeurosis covers the Rectus Abdominus, so we wouldn't see the Rectus Abdominus's edge if the obliques's layer is visible on top of it (unless you'd wanna draw the obliques with "low opacity" :) ). See images attached - Hope this helps clarify your doubt! 2. For your second question (ribs X obliques), yeah, it really is a bit confusing! But a good reminder could be: if the torso isn't bending forward, laterally or twisting, then the obliques are not active/flexed, so this increases the probability that ribs might be seen on the surface if the person is skinnier. About getting the origins and insertions right: I believe one thing that might help is to have in mind what "approach" you wanna have with your anatomy studies: 1) you can do a more observational sketch, where you really draw from the photo identifying the anatomical parts you see (that can be useful for the kind of "quick sketching" studies); ...or 2) you can do sort of an "x-ray" type of study, where you don't rely too much on the reference to draw what you see, but instead just use it as a starting point for the pose, then draw all bones and muscles as visible and based more on your knowledge of the anatomy than on the reference photo itself. Both ways are certainly useful, but personally I think the second way is a bit easier and more "beginner-friendly" because it doesn't require you to recognise everything you see on the photo (sometimes THAT is the hard part). I also think it's more effective when we're learning the basic anatomy because it gives us the opportunity to directly apply the actual anatomy knowledge itself (rather than to draw from observation). So, to increase the chances you'll get the origins and insertions correctly, I'd suggest you try doing anatomy studies the "x-ray way" first when you come across this kind of doubt - use the reference just as a starting point, but rely more on applying the anatomy knowledge from your memory than on drawing from observing the photo. Then, once you get familiar with the basics, we can start blending that knowledge with your observation skills and look for varitions on the surface in different models, body types and crazy poses. Hope this all helps.