Well I wish I understood this but I'm sorry I don't. They do look well made though.
Maybe when I get a router table setup it will make more sense to me.
These are for making rounded corners on flat materials, like the corners of a table for example. They are really templates that are attached over (or under) the corner of the workpiece. You can then use the template to draw a line so you can cut most of the corner away (band or jig saw cut just outside the pencil line.) You then use a "trim" router bit with a bearing, resting the bearing against the template's curve. This bit cuts to the exact shape of the jig so you get a good edge and a perfect rounded corner. He made a number of them.
The aluminum L bar is deeper than the template is thick so they hold the template in place on the workpiece, but are set back far enough to be out of the way of the router trim bit.
He made many sizes so he can use them later. It is always easier to make all of the same type of jig at the same time.
Cutting rounded corners freehand is basically impossible. No one is steady enough to do that. Jigs like this take a lot of careful cutting, then sanding to the final shape.
You can buy plastic circle drawing templates with marks every 90 degrees to line up on the corners of the jig material. Draw the line, then cut and sand the jig, attach the aluminum L brackets, drill a large hole so they can be stored on the pole device in the final picture.
Very smart way to do this. You could even use double sided tape to hold the jig in place a little more firmly. But the pressure of the trim router bit would probably hold it in place quite well without the tape.
The more you get into woodworking, the more you will like jigs. They make good results easier and make matching parts a snap.