Structures and mechanisms are two of the labor-intensive parts of design.
Mechanisms design is the art of mechanical engineering, where you design the moving parts -- door hinges, latches, axles, landing gear, mechanical attachments, bracketry, and all those fiddly bits that make a spaceship go.
The structures are the backbone of the spacecraft. Besides the obvious design of structural elements designed to carry mechanical loads from one place to another, the structures guys analyze the structural requirements of the integrated spacecraft. For example, that design I did showing 6 hydrogen tanks is not the lowest mass you could use if your only goal is to contain a pressurized liquid; but there's a strong argument for having those skinny tanks around the outside because of their contribution to the structural integrity of the whole vehicle.
We group them together because the structures and mechanisms have to work together. Every moving part has to have something to react its forces against, and those forces have to be carried by the structure. Mechanisms are to structures as software is to computer hardware.
Whenever it's feasible, you want to survey the world of existing structures and mechanisms, and use things that are available off the shelf. Around NASA the most popular places to look for odd mechanisms are boat shops and hardware stores. You'd be surprised how many sailboat parts wind up in a spaceship!
Now to lay out a general task plan for the Structures and Mechanisms Technical Committee:
It makes sense to start at one end and work the way to the other. From the moon up, we have ...
the landing gear pads,
the main gear struts,
the attachment between the pads and the struts,
the attachment to supporting struts,
the supporting struts,
attachment of supporting struts to descent stage primary structure,
descent stage primary structure,
and so on...
This is just an indented list showing which parts go into which assemblies, which in turn make up larger assemblies, and so on until we've got to the top configuration drawing for the whole, integrated spacecraft.
It looks something like:
Drawing Number Item 1C10000000 Reference Mission Spacecraft 1C11000000 Lunar Transfer Vehicle 1C11100000 LTV Command Module 1C11200000 LTV Service Module 1C12000000 Airlock 1C13000000 Exploration Base Habitat 1C14000000 Descent Propulsion Stage 1C15000000 Ascent Stage
It's a whole lot like figuring out what parts you need to put together to make a rendering of the spacecraft, except that you just keep going to identify even the parts you can't see.
I used "1C" drawing numbers to identify conceptual design. We could use 1T for test configurations, and 1F for flight. I swiped that system from McDonnell Douglas. Every aerospace company does it a little differently; but what the heck, MDC has managed to get a few airplanes and spaceships off the ground so we know the system works.
At first designing the fiddly bits might seem easy. In many cases, it is; but it still has to get done so that we know what we'll have to either buy or make. As the designs come together, we can start adding up the weight of this thing. Most of the dry weight of the spacecraft will be structures and mechanisms. Interestingly, you'll find that fasteners can weigh as much as the structures they fasten.