For a real arm this is not trivial, because the skin stretches and also forms wrinkles. To achieve a realistic representation, one would have to measure the arm in some intermediate positions and store the data and then set and connect certain vertices accordingly.
I have only created some primitive joints.
See from the Collection of examples from discourse.threejs.org :
DynamicTubeGeometryCaps
Pino
CustomCylinderAndKnee
CustomCylinderAndSphereKnee
BendCylinderToKnee
BumblebeeMara