Before you start detailing the lower torso, sand everything thoroughly. Smooth and even surface will help all the small details to show up vividly. I start this process by drawing centerline, curves of the belly and the belly-button.
At this point, you can decide all the features that your doll will display. You can sculpt abs or nice pouch for a squishy belly, leave the torso sleek and only carve a belly-button. It's up to you how much details you'll add!
The ab sections in humans vary greatly. They rarely are symmetrical so you can create any variation you want. When sculpting the abdomen I like to distinguish the centerline, which is the tendon dividing the abs into left and right halves. After that, I'm just adding more clay to the lower belly section and define abs a little bit.
Generally, women tend to have a higher body fat percentage than men, because of that the abdominal muscles will be less pronounced and visible mostly around the centreline. While male abdominal muscles can be more pronounced not just in the centre but all around the rib cage making the abdomen more square-shaped.
If you want to steer away from generalizations, you can sculpt your doll without pronounced abs or make the physique extremely fit and muscular. Here are a few examples of different abdomen shapes for the reference.
Once you're happy with the shape of the abdomen, leave it to dry and sand the surface thoroughly. Now it's time to carve the belly-button and make any other small changes. When you're carving only the surface layer of clay wet the area a little bit. In the following video, you'll see how I carve the belly-button area using metal sculpting tools and a damp brush to keep the surface wet.
For the lower back, I keep the detailing minimal by carving the indentation for the spine and making lower back dimples, that sometimes are called "dimples of Venus". As usual, I start by drawing all the changes I'm going to make.
Not delving into muscle anatomy too much, lower back muscles form two columns on each side of the spine and a depression in the middle. The depth of the spinal depression depends on how developed the muscles are.
I'm using the same wet surface carving technique. If you think that you need to make bigger changes and carve deeper you can use scalpel as well.
Because there are no muscles that cover the spine in the lower back area, you can make little indentations to resemble the spine vertebrae.
For the last step, sand the whole piece thoroughly with a very fine sandpaper.