These used to be sold as “corner blocks/squares”, but these days they are in the Carbide 3D Print Library:
https://carbide3d.com/3d-print/corner-square/
and an older version was drawn up as a file:
I believe this sort of thing is discussed in the video:
but basically, it’s a combination of:
- securing a blank to your machine using some suitable workholding: Workholding - Carbide 3D
- setting zero relative to it: Movements and Zeroing
- drawing up a contour or pocket and assigning a toolpath to cut away the recess: Hello Contour or the following section
but pretty much any approach is valid — my recommendation is to get a couple of pieces of scrap and try out different techniques until you arrive at one which you feel suits how you wish to work.