BCnF (build, certify and fly) hobbyists make their own airplanes from scratch or out of kits, extensively customizing them, and then get them certified by the FAA and take them into the air. The US has more than 32,000 registered homemade airplanes. BCnF makers produce everything from racers to fabric-covered biplanes. This post is a good introduction to the fascinating world of BCnF, and is a great place to start if you're thinking of kit-bashing your own flying machine.
Old-fashioned plans-built airplanes take thousands of person-hours to build: wood has to be cut, sheet metal has to be bent, steel tubes have to be welded, fiberglass has to be laid up, and so on. Kits with pre-formed, pre-welded, pre-molded, and/or pre-machined parts then brought the build time down to the range of approximately a thousand hours. These are the most popular today, and include the Glasair 2 being built by the ONE BCnF team. The most modern (and most expensive) "quick build" kits, however, come with pre-drilled holes and other features that make it possible to assemble the airplane in just a few hundred person-hours. The creators of these kits sometimes even allow customers to visit the factory to build their airplanes under professional supervision. (Thanks to Lean manufacturing principles, Glasair has managed to get their build process of their latest kitplanes down to two weeks of full-time work. To the many thousands of people who have spent years putting together a homebuilt airplane, two weeks is nearly unbelievable).
Because they are not built by professional mechanics/machinists, these kit-based (or made from scratch, based on plans) home-built airplanes are certified by the FAA as "Experimental, Amateur-Built", or EAB.
There are currently about 32,000 homebuilt airplanes registered with the FAA.
The largest manufacturer of airplane kits is Van's Aircraft, creators of the RV series. There are over seven thousand RVs flying out there, over a third of which are RV-6s like mine, the most popular EAB out there. Next on the list is Glasair, who has delivered over three thousand kits. The Glasair 2 is the most popular, with over 1,200 flying. Many other kitplane companies each have several hundred airplanes flying, built in garages and basements and small airplane hangars around the US (and in a few other countries).