Basically, it’s an addon to the bed leveling procedure using a sheet of paper. He further refines the process by printing small, 1-layer squares at the corners and center of his bed. I tried it and found it really makes a difference. Since you have a BL Touch, this could be a way to verify your BL Touch is fully dialed in.
Yea i did the 100 mm extruder test when i replaced the stock plastic extruder on my ender 3 v2 with a all metal one. It was dead on…I replaced it because the plastic one broke when I was replacing the filament. Broke underneath so you couldn’t see it but could feel it give when i push the lever in to remove the filament.