Video has a diagram: A “threadless” system refers to a system where a fork with a non-threaded steerer tube extends through the head tube. A stem (A) then clamps around the outside of the steerer tube along with spacers (B) if necessary. Finally, a top cap (C) keeps everything in place and acts as a headset adjustment.

This article covers the full process of how to replace threadless stems, including choosing compatible components, proper spacer orientation, and final centering and headset adjustment.

This video is by Truman at Park Tool.

Starts out showing various way how the stem clamps onto the steerer tube.

Quality:
  Has good diagram showing how it goes

What controls the clearance? I think it is just where you clamp the stem, once it is preloaded with the top cap.

1:23 Handlebar diameters:

   22mm
   25.4
   26.0
   31.0
   35.0

Common Steer tube diameters:

  1"
  1 1/8
  1 1/4
  1 1/2

 2:00 Stem Angles and length, Spacers