The picture of the white corner block works the same way as the 3rd. picture, it's holds the sides and the back or front of the cabinet/table/workbench/etc. sq. , then you drive in 2 pocket screws to hold the cabinet parts together using the pocket holes with just a bit of glue in the slots, then once it in place and the top is put on you drive a screw using the hole in the center of the corner block, this will hold the top in place and keep the cabinet sq. all with just one block.
I like to off set the tongue "off center" this will let you flip the corner block over to get it just below the edge of the cabinet parts,this will let you pull the top down if you have a small gap in the parts. (the off set is only a 1/8" the norm)
The corner blocks can be anything you have in the shop (scrap) unlike the ones in the 3rd. picture.
The push block is used to put on the Tongue on the corner block, because the block is cut on a 45deg. it's a bit hard to hold it true to the fence when you make the pass by the router bit, but the push block will make it safe and easy to make the corner blocks.
I like to use the set below because it will let me use up to 1 1/4" thick stock, the norm is 3/4" max thick stock when using T & G sets, I also use the CMT set all the time because it will put in a 1/4" deep slot/tongue and the norm is 7/16" deep slot but both sets work well.
MLCS Two Piece Tongue & Groove Set below (OK set below▼
Also this one at a lower price
Tongue & Groove Assembly, same web page
Grizzly T & G set ▼ below ( one arbor/shank type) with shim washers.
CMT set below ( the best ▼
With the bearing below you can put in the 7/16" deep slots if needed for
cabinet door frames....
will fit the same arbor....