Yeah, I can't see anything in that video.  Use screen capture software instead of your phone and turn the gizmos off.  There's a giant camera gizmo covering his shoulders.

The problem is very simple, you have too many possibilities. You need to reduce the amount of possible IK solutions.

Constraints should be first. Human arms have limits that reduce possible movement. Second should be pole vector. This is some kind of best case solution, often targeted by the elbow or knee, that the solution should strive for.

With pole vectors and constraints you will be set for basic animations like this.