The hinge joint in unity is basically used to connect two rigid bodies. When there is a need to constraint the movement of an object
then we require to add hinge joint on it.
For eg - Door,Pendulum.
In this we restrict the movement of objects to certain limit. So in order to connect them, we use hinge joint.
Hinge joint is applied to a gameObject.
To connect two rigid bodies with hinge joint, Add hinge joint on one body and assign the other body as Connected body.
Hinge joint has many properties, According to which we can modify hinge joint as per our requirements.
Anchor is orange color arrow, it actually shows the point where two bodies are connected. They will move around it.
We also need to specify the axis(x,y,z) along which the connected bodies will move.
Below is example image of Hinge joint-
Fixed Joint in Unity3D
The fixed joint in unity is used to join two bodies when we want to restrict the movement of an object dependent on each other.
If we want to fix two bodies permanently then we can use this joint in this scenario.
In fixed joint it is not necessary to assign connected body like in hinge joint.
It requires rigidbody.