There are numerous forms of robot structures that can be done in many ways. A lot of vendors have different structural components. What our team typically does is utilize 2x1 or 1x1 aluminum tubing in tandem with gussets. We then drill into the 2x1 tubing to put in bearings and attach components or other structures
The necessity of a belly pan depends on what drive train the team uses and/or how the team wishes to build the robot and/or how the team wishes to do the electrical board.
Belly pans act as one giant gusset at the bottom of a robot which makes the base structure of the robot really rigid and it protects any underside components when crossing over obstacles.
The creation of belly pans and the design of them depends on the team's machining capabilities/access.
The drive base is the backbone of your robot, it's where your drivetrain goes, where everything (bumpers and superstructure) is mounted from. There is a COTs option available to rookie teams but what we typically do is use gussets and some 2x1 aluminum tubing
This is basically where all of your mechanisms are, it's everything that is mounted on the drive base that is not the bumpers. There is no perfect superstructure construction method and it always varies on the game and the mechanism.
A nice example to see how robot build constructions can look is through YouTube and CAD files, our Robot CAD files will easily be found once you join the team's Onshape. The video below was a starter bot design by REV robotics that was supposed to give teams inspiration and ideas on how to build their bot