There are some really bad examples of frame jigs in some of the DIY minibike and scooter sites...
I won't name them because the guys who did all the work for the sites were just minibike enthusiasts trying to promote the hobby, and not industrial engineers...
A simple fixed/non-adjustable frame jig can be made of plywood if someone wants to make a few frames that are all the same material and dimensions...
An adjustable jig made of steel allows changes in frame geometry and size...useful for producing numerous frames of varying size and style...
Skilled builders can make 'true' frames without a jig, using a build bench, full scale drawings, and clamping/aligning tools...useful if you build custom frames of odd materials, and rarely build two that are similar in size or shape...
The type of jig best suited to someone's needs depends a lot on what they intend to use it for...