There is no doubt about it, the shoulder is a complex joint. So much so much so that there is a movement among some Alexander Technique teachers to abolish the shoulder entirely! While it's true that the arm begins at the chest and extends to the fingertips, I think the complexity of arm and torso movement is awesome and can be appreciated in its own right.
Three in one - anatomically the shoulder consists of three joints involving the sternum, clavicle or collarbone, scapula or shoulder blade and the humerus.
The only bony connection to the torso is between the collarbone and the breastbone.
The bone of the upper arm - humerus, connects to the shoulder blade which floats on the back ribs and it is that scapula bone that connects with the clavicle.
I think it's beautiful how these bony connections form something like a chain that allows an amazing range of movement.