Two spherical 4R mechanisms are connected back to back.
4R: 4 revolute joints.
In each mechanism the center lines of 4 revolute joints intersect at a common point.
The angle between center lines of revolute joints for the orange link is not 90 deg. (rather than Cardan joints).
Angle between the input and output is A = 90 deg.
Angle between cylinder of the orange link and the input shaft is B = A/2 = 45 deg.
This condition makes the mechanism a constant-velocity joint.
The orange link rotates without fixed bearing.