This is a joint which can be used as a knee or elbow to add articulation to a figure. I've used the ball & socket joints in tinkercad for shoulders and hips, but I wanted to experiment with a bending knee joint.
These two pieces snap together (you have to push very hard, be careful not to pinch your fingers or break the thin part), and once snapped, they should stay together and bend quite well.
I have built legs and arms around them in a full figure, and they work well.
I have included the pieces together and separately in case you want to import them individually.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/dAhFOyjXgnU