Conceptually, they are a smaller square tube inside a larger one, with four rubber rods in the corners.
Here's a frame design lifted from a 1952 Silver Streak Clipper I used to own. A center tube (this one is large for water storage) with C-channel cross-members. Each cross-member has a carriage bolt through the deck (not shown). With a central spine you can 'tub the wheelwells' to your heart's contentment. It does need a monocoque shell for torsional rigidity.