You can see a number of CAD screenshots of this year's drivetrain
here
Nothing is inside the tubing itself. It's just two halves of a bearing housing that slide into each other and then ride inside the pocket in the frame. You can see specifically how this works in
this image. On the inside of the frame there is a sprocket mounted on the shaft to drive the wheel. The center bearing housing goes into the frame, and then bolts to the actual transmission plate.