I have this character, for the last hour I’ve been trying to put their hands behind their head like they’re laying on a bed relaxed… But the arm and shoulder mesh stretches…I can see that the reason is because the weight painting isn’t perfect. I’m not sure the exact style and gradient of weight paint to use. Can you look at my blender file https://drive.google.com/file/d/1rY4CXTg5eZzdUuZL8P6LqWTX8hYlaXVC/view?usp=sharing and paint for me how it’s supposed to look so I could put the characters hands behind their head like he’s laying relaxed on the bed like this
and not stretch the mesh… Then I could have a solid professional basis for basing my view on weight painting correctly…Thanks in advance. 
You may need more than just weight painting to get shoulders to deform correctly, because they don’t deform in a physically consistent way. Check yourself out in a mirror and move your shoulders around, and you’ll be able to see that it’s an extremely complex joint. There are many tutorials and add-ons about using drivers to get better shoulder deformations- my personal favorite choice is https://blendermarket.com/products/rbf-drivers .