You're probably over-thinking things a bit.
When you use a custom white balance, you take a photograph of something that should be "white" if the light source is also white. Most light isn't pure "white" and has a bit of a color-cast to it... possibly causing things to look slightly yellow or orange... or blue ... or even green. Anyway, if the camera knows that the object being photographed is supposed to be pure white. then it can look at the pixel values and notice that... there's a touch too much 'blue' in them... and it can adjust to compensate for the color cast. Having done this using a target sample, and knowing that everything else you shoot is in the same light, it realizes it would need to subtract the same amount of 'blue' from every pixel in every shot you take. That's the point of white custom white balance (and as soon as you changing lighting conditions to some other light source, you have to re-shoot the custom white balance frame.)
The easy solution is to just get close enough to the white balance target so that it fills the entire frame (corner to corner).
But the point in showing that example (showing a region with a giant rectangle made with those dotted lines) is to show that the white balance target doesn't really have to fill the entire frame... but it does need to dominate that area in the center.
The pattern of small boxes you see is simply an indication of where all the auto-focus points are in your 80D camera (when shooting through the viewfinder... not live-view mode). Those focus points are there whether the boxes are drawn or not.
To change focus modes, you normally press the AF pint selection button (that's the upper-right coner on the back of the camera) and that lets you change AF points within your current AF area. But while in that mode, you can press the tiny button in front -- near the shutter button -- and that lets you change AF areas (that's how you toggle between modes such as single AF point, surround, full auto-select mode, etc. etc.) This is covered starting on page 114 of your manual.
Make sure you read your manual on how to do set AF areas and AF points. You can find all the manuals here:
https://www.usa.canon.com/internet/portal/us/home/support/camera-user-manual
Tim Campbell
5D III, 5D IV, 60Da
