Here are some close up of different ways of connecting tube
This picture shows taking a round tube and inserting it into a square tube with a slight press fit. A star nut secures the tube.
This picture shows both a bolt through connection and also attaching IGUS bearing blocks for a pivot point.
This picture show both rivet connected sheet metal but also a tube being used as a linear slide.
This picture shows tubes being connected together using a tube connecting nut (we call them star nuts).
This picture shows a custom FDM bracket used to connect tubes together.
I hope the pictures help let me know if there is anything else you would like to see or know.
Allison