This is an interesting hardware piece addition for 20x20 aluminum extrusion. Rather than being a 90 degree type plate, its a hinged piece that connects the end of one extrusion to the side of another, but with any angle you wish. You can loosen/tighten the angle with the screw on the side. Given its just a bolt, it's not incredibly strong so while you can use it for structure, its not a load-bearing connector: make sure there isn't a lot of torque on the joint. You can use thread-lock to make it stronger.
The half that connects to the side one an extrusion simply uses a single M4 or M5 x 8mm long screw + T-nut as you'd use with any other plate. For the half that connects to the end of an extrusion, you'll need to tap the 5mm diameter center-hole of the extrusion and screw directly in. We don't include a tap or matching screw so you'll want to pick those up from a hardware store.