Need mathematical explanation

I looked at his code a bit more he basically wants to know how to get the y angle (horizontal sorta) between the vector formed from the point to the players head and the players torso. So lets say the player was facing directly at it, it would mean that his returned value would be 0, but lets say it was on the side of their ear, the y rotation would be 1 on the graph***

this would be 1 meaning we have to rotate it by 90 degreesScreen Shot 2021-10-17 at 9.20.17 AM
this is 0, they are already aligned so no need to fix aka 0 degreesScreen Shot 2021-10-17 at 9.20.55 AM

ok so i kind of understand what does 1 mean in the Unit Circle but how does that equal to the radian angle or i misunderstood it again

And can you visualize the Y component of a cross vector in 3D environment following x,y,z axis? It is for me to easier understand how the Unit Circle form in the 3D environment