Fix one part as the assembly's base, add joints so other parts can move against it, check that nothing collides, and animate the whole robot through its motion.
An assembly combines multiple parts into one model -- insert parts, then define how they connect. Always ground exactly one part (usually the chassis) as the fixed anchor; multiple grounded parts break motion simulation.
Joints define how parts move relative to each other; constraints lock parts that should stay fixed.
Each joint type controls which directions of motion are allowed. The two most common in robotics:
Insert and lock your base component first.
Add parts one at a time and constrain each immediately.
Drag parts to verify joints, then run interference detection.
Ball Bearing -- assembled with concentric and revolute constraints.
Robot Chassis - the grounded base component that anchors every assembly.
You have a two-wheel differential drive robot. Plan the assembly constraints needed.