So the answer to the question is actually present in what DIO had stated before, but sometimes it's necessary to explain the same thing a few different ways before we find one that helps the student to understand.

One of the standard steps we apply to most of our animal constructions is taking the ribcage and pelvis and using them as the basis for a larger torso "sausage". In doing this however, the sausage swallows up the ribcage and pelvis masses, leaving no trace of those initial masses to protrude. If they were sticking out from the torso sausage, then we'd certainly have something for our later masses to "wrap around", but since the ribcage and pelvis end up blending smoothly and seamlessly into this new sausage mass, there's nothing left of them to wrap around. The only surface for our new masses to attach to is that of the sausage.

When adding masses to our construction, what we're focusing on is the structure as it exists at that moment - where is it flat, where is it rounded, where does it have parts that stick out, etc. - so we can understand how this thing exists in 3D space right now, in order to build upon it.

Think about it as though you were building something with clay. First you make a few balls (for the head, ribcage, pelvis), then you make a cylinder to connect the ribcage and pelvic masses into a big torso sausage - making the ribcage and pelvis themselves disappear into it.

I should mention that we mostly only end up doing this for the torso - in most other cases we're not really "swallowing" one form into another. But what matters here is that we're focusing on how the 3D structure you're building up exists in space at any given time, which parts stick out, and which parts are smooth.