BM's ''large circular bit'' on a traditional spinning wheel is simply a pulley wheel used to turn a small circular bit (called a 'flyer') much more quickly. The ''large circular bit'' won't be found on an electric machine.
Have a look
here. Ignore the first few items, and scroll down to ''I won a Blue Ribbon 1st prize...'', and play the video.
You'll see that the yarn is fed into a tube, then out of a slot in the tube lined up with a hole in the boomerang-shaped piece of wood. (You'll see this best about halfway through the video). it is then guided over the two horizontal pieces of dowel that stick out from the boomerang bit, and these wind the twisted yarn onto the bobbin.