Not quite sure what you're asking. They make specific fasteners for the extruded aluminum and are usually button head machine screws and t-slot nuts. If the entire frame is 80/20 then you'll need corner brackets or plates and the above to assemble, or you can tap the 80/20 end parts and drill an access hole for an allen wrench use the screws and t-slot nuts that way. If you use aluminum plate for the ends or gantry sides, then just tap the 80/20 ends for the button head machine screws you're using and mount the plate to the 80/20 as needed.
It all goes together fairly simple with the extruded aluminum. Hunt around the forum for photos of others. I used a welded steel table to mount an 80/20 framed machine to I built. There are some photos in the gallery under coherent. If you have any specific questions just ask!