From single parts to working mechanisms
A single CAD model of one bracket is useful, but a robot is made of many parts that move relative to each other, and CAD tools have a separate mode for this called an assembly. An assembly brings multiple individual part models together into one file and defines how they relate to each other: which surfaces touch, which axis a part rotates around, which faces stay a fixed distance apart. Those definitions are usually called mates or constraints, and they're what turn a pile of separate models into a mechanism the software understands as connected and movable.
The gripper you'll design conceptually here is the same kind of mechanism covered in mechanical depth in Robotic Grippers and End-of-Arm Tooling, and the joints in a robot arm work the same way conceptually as what's covered in Robot Actuators Deep Dive. This lesson isn't about redesigning either of those, it's about the CAD process that lets an engineer check, before anything is built, that the parts making up a gripper or an arm actually work together as a mechanism.