Mentioned below are the most common methods of constructing flats. They all have pro's and con's for their use dependant on budget, resources, frequency and intended usage of the flats and as such you can expect to come across any number of adaptations and permutations of these basic construction methods.
Steel is commonplace in flat construction and although the principals are similar their construction involves different construction techniques and fabrication processes.
Timber and sheet materials
Here are some sites you may wish to look at, although some of them may not be directly related to theatre production, they will have some tranferable elments.
Festool This site is good for downloading demonstrations of the products.
Finewoodworking You may have to sign up to the free trial period to access the demonstration videos.
Flints This may be helpful for descriptions of specialist theatre hardware and costings.
Capstain table just for fun, a really cool table.
Band saw magic - that guy got skillz, yo!