Those pics were screen shots taken from a youtube video on how to install the handlebars on an engwe 750 pro. This is not me in the video lol.
There is a video on the how to install the hidden extender on this page...
Amazon.com : NIUAWASA Bike Hidden Stem Riser Bike Fork Stem Extender Bicycle Handlebar Raiser Adapter Head Up Adapter Aluminium Alloy Suitable for Road Bike MTB BMX Fixie : Sports & Outdoors
I just measured the orig engwe stem, the upper stem bolt center line "is" in line with the top of the steerer tube, just like my drawing at 21mm. The internal part that surounds the steerer tube and you place the top cap on is 24mm tall. So that results in a 3mm extention for head stack tensioning. Go figure lol.