This is partly right. Density is mostly independent of the softness or firmness of a foam material (both low and high densities can be made in a full range of firmness and softness levels) although they are very loosely related (in the same “variety” of foam … then much firmer is often a little higher density than much softer but not always). Density though is the single biggest factor in the quality and durability of polyfoam and memory foam.
Density is also related to compression modulus which is the rate that a foam gets firmer with deeper compression. Higher density foams tend to have a higher compression modulus (also called sag factor or support factor) although this too is not a “rule”. The highest quality of polyfoam which is called HR foam has to have a compression modulus of 2.4 or higher (the ratio between the 65% compression weight and the 25% compression weight) to qualify as HR foam. When a foam has a higher compression modulus … then even if it is softer than another foam with surface compression it will get firmer faster and can be more supportive than firmer foam with a lower compression modulus. Compression modulus is also a key part of the comfort of a foam as well because in combination with ILD (AKA IFD) it is part of what determines the depth of the cradle which is what redistributes weight over a larger surface area and relieves pressure.
Pressure relief is a very complex subject but in it’s simplest terms it involves spreading body weight over a larger surface area so that the pressure on any particular part of the body is lower than the pressure inside the capillaries so the blood continues to flow. There’s a very detailed and technical discussion of pressure relief in post #2 here.
As it relates to a material though … pressure relief (and the corresponding sensation of softness) is connected to ILD/IFD, Compression modulus, and Point elasticity (where memory foam and latex excel and which determines how exactly a material can mirror a certain shape) which along with the thickness of each layer (which is independent of the properties of the material itself) controls how large a surface area is bearing weight and the distribution of the weight. Some types of zoning can also even out pressure by allowing more sinking in of some areas which allows the more recessed areas of the body to bear more weight.
In the case of memory foam it can be even more complex because it’s softness is also affected by temperature, humidity, and the length of time it is continuously compressed so how soft it feels can also depend on the temperature sensitivity of the memory foam and can vary with your environment and circumstances or by how quickly a particular memory foam becomes softer with body heat.
Pressure relief also needs to be balanced with support and alignment because you could use a very thick slab of low ILD and support factor foam to do a great job relieving pressure but the heavier parts would sink in so far that the spine and/or joints would be twisted, bent, or flexed outside of their neutral range this would lead to the pain that goes with misalignment of the spine or joints.
The benefit of convoluted foam is first that it is less costly (you can use one layer of foam to make two convoluted layers that together are slightly thicker than the original layer) but it also alters the feel and performance of the foam. The shape and depth of the convolutions would mean that the upper surface of the foam was softer (less material) and as you sank in more into the thicker parts of the convolutions it would become firmer faster and then as you sank into the non convoluted part of the layer it would become firmer faster yet. In other words … it simulates a higher compression modulus which means that it can be softer on top of the layer and firmer in the deeper parts of the layer. The tradeoff is durability compared to a solid layer of the same type, density, and firmness level of the foam although this wouldn’t be a significant issue with more durable types of foam such as latex. This is a common part of mattress manufacturing in Europe where you often see special fabrication of various layers with surface modifications and various shapes and cutouts and convolutions to modify the feel and response of the foams used. It’s also becoming more common in North America.
As you can see this is all quite complex but again the three specifications of a foam material that control most of the pressure relief (along with layer thickness) are ILD/IFD, compression modulus, and point elasticity (and temperature sensitivity with memory foam) which together with the thickness of the softer layers create the pressure relieving “cradle” and needs to be balanced against it’s “opposite” which is support and alignment.