It is basically just a tube with an appropriate cross section and volume which is cut to the appropriate length for the intended rpm.
If you are familiar with the way people build moose can pipes, it is not much different in theory. Similar too was the exhaust extractor used on the Formula 1 engines as well.
Key design work involves enclosing the device so it gets a bit quiet, and so it develops the best balance of backpressure for the engine.
And most important, the assembly of the muffler is rather involved. It took a bit of time for both Nelson and Jett to come up with assembly processes which stayed together. Dub's process has evolved to create a very robust assembly. Very rare one comes apart any more. About 1 a year
Bob