The best robotic arm has 7 degrees of freedom, not 6, why?

The best robotic arm has 7 degrees of freedom, not 6, why?
The most commonly used industrial robots are generally six-axis, but the recently launched man-machine collaborative manipulator has 7 degrees of freedom, and I have been wondering why. Until recently, I saw a question on Zhihu: How many degrees of freedom does a human arm (wrist joint to shoulder joint) have? It was discovered that the original 7 degrees of freedom are a true restoration of the human arm.
7 degrees of freedom.
Someone asked if 5 and 6 were the same. 5 is the only joint to rotate when turning the key, and the power comes from the torsion of the two radials of the forearm; 6 is the only joint to rotate when the mouse is placed on the desktop, and the power comes from the rotation of the wrist.
As for why human arms have 7 degrees of freedom instead of 8 or 6 degrees of freedom, it may be because God knows robot control very well. Let me try to briefly introduce it below.
First introduce a theorem:

A manipulator with 6 degrees of freedom cannot change from one configuration to another without keeping the three-dimensional position of the end mechanism unchanged in space.
This theorem is not easy to understand at first glance. Consider a simpler case:
Similarly, for a manipulator with 6 degrees of freedom, even if the three-dimensional positions of the end mechanisms corresponding to two sets of configurations are the same, the manipulator cannot keep the end mechanisms still when moving from one configuration to another.
If someone has watched industrial robot welding things on TV, they will find that when it is welding in the same position, it will twist to this side for a while, and then twist it to that side for a while, it looks very cool.
In fact, this is only done because, although welding only wants to change the orientation of the end mechanism without changing the position of the end mechanism, due to the limitation of the theorem, it must be moved back a little and then twisted in various ways to ensure that the end mechanism moves It will not hit anything during the orientation, because the three-dimensional position of the end mechanism will definitely move when moving.
Think about the action of turning the key when opening the door. In this case, the three-dimensional position of the end mechanism (hand) of the human arm has not changed (always in front of the keyhole), but the three-dimensional rotation of the end mechanism (hand) has changed (the key is turned) . Humans can achieve this simple movement because our arms have 7 degrees of freedom.
Having said that, the inspector may see it, hey, I understand, my end mechanism has 6 degrees of freedom (three-dimensional position, three-dimensional rotation), and the arm as a manipulator has 7 degrees of freedom, these two freedoms Degrees seem to be different, but the number is 7-6=1, so I can use this 1 degree of freedom to turn the key.
If God designed our arms to have 6 degrees of freedom, the action of turning the key would be very exaggerated. You can feel it without turning your wrist when turning the key.