The structure in the picture is the pressure vessel of the Dragon (blue in the picture below). What you have labelled
A is the forward section, containing the crew couches. The part labelled
B is the aft equipment bay, which stores equipment and supplies (green in the picture below) which are accessible within the cabin.
The Crew Dragon is a truncated cone, a popular design which started with the block II Apollo command module
continues with the SpaceX Dragon and Boeing Starliner crew modules
and is proposed for the future Russian Orel crew module.
Truncated cone crew modules are built in two layers. The outer layer is the heat shield (aka thermal protection system). The thickest and most heat-resistant part is the "aft heat shield" at the bottom of the cone. However, the entire outer layer is heat shield, just not as thick as the aft heat shield.
The inner layer is the pressure vessel. This is an airtight compartment which contains the crew and any equipment or supplies that they must be able to access within the cabin.
The apex (tip) of the pressure vessel layer is truncated further back than the heat shield layer. This creates an unpressurized forward compartment which (depending on the spacecraft) contains the docking tunnel, parachutes, uprighting bags, and landing beacon.
The base of the pressure vessel usually stops short of a full cone, ending in a cylinder (which you have labelled
B). There is no hatch between these two sections, but crew couches help to partition the space. The space between the cylindrical part of the pressure vessel and the conical heat shield creates an unpressurized aft compartment, along the "bottom edge" of the cone.
Crew modules have engines to adjust the spacecraft attitude; these are typically placed in the forward and aft compartments. They may also have engines that act as a launch escape system, initiate de-orbiting, or provide a powered landing; such engines are typically found in the aft compartment. The aft compartment also contains storage tanks for the fuel, oxidizer, and pressurant for these engines, and reserve oxygen tanks for the crew. The Apollo aft compartment contained crushable ribs, which helped to absorb the impact of landing. The ribs on re-usable spacecraft are far more sturdy. The Crew Dragon has landing legs, which deploy from the aft compartment.
In between the two layers are framework called longerons, which provides structural integrity to the capsule and transmits forces. The side hatch must cover openings in both layers. In the original Apollo design, each layer had its own hatch, which made it more difficult to extract the astronauts during the Apollo 1 fire. Modern hatches seal both layers with a unified hatch.