Sealed bearings are a common component on modern bicycles.
Even if you take off the plastic covers of sealed bearings, the ball bearings inside will not fall out.
How is such a part assembled in the first place?
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Usually they are pressed together with a hydraulic press. Also, given the ball, count there can be sufficient space for the balls to be placed in one half and then spread around the periphery and held in position with the spacing device - which can be plastic or metal and in one or two pieces which get glued, welded or riveted together.
Some bearings are designed, like taper roller bearings, to disassemble easily.