Another thing that you could’ve done was to use the Child Of constraint in blender and set it to the specified bone. Then, pressing the Set Inverse button.
For the arm bones to follow the torso bone when rotated, they need to be set to parent. You should then see the arms being under the torso in your hierarchy. (Click the child then the parent and press ctrl-P)
For the mesh to properly follow the bones, like cloudy mentionned, you need to parent them with automatic weights to your armature, multiple meshes can be parented to an armature, but sometimes it’s easier to apply the weights as one and then split them after (if you need them split in studio) I’d suggest looking for a tutorial on skinning and weight painting to get a better idea of that this works.