The Extra series of aircraft are built in Germany by a company called Extra Flugzeubau. The founder of the company, Walter Extra, started off with an Extra 230, a 4-cylinder Lycoming engine and while the airplane was well tested, it was certified "Experimental." Walter's next aircraft, the two-seat composite Extra 300 (a mid-wing design) was also certified Experimental, though I believe some of those have been certified in the "Standard Aerobatic" category. The next two airplanes that evolved the lower mid-wing design Extra 300S (single seater) and 300L (two seater) are both fully certified "Standard Aerobatic" category airplanes.
All of the Extra aerobatic airplanes have used Lycoming engines and all since the Extra 230 have used 6-cylinder AEIO-540 (300 HP) engines.
That being said, my airplane and several other Extra 300S models are certified in the "Experimental Exhibition" category. While this category adds a few more restrictions (you cannot take passengers for hire, there are some restrictions as to where you are allowed to fly, and you have to give the FAA a "program" letter of intended airshows if you are an airshow pilot), it also can give some latitude in making modifications to the airplane. For example, my "Cirrus Extra 300S" is certified "Experimental Exhibition" because I have a non-certified Lycoming AEIO-580 engine on it, as well as a bigger rudder. Being experimental I can also use a camera mount on the wing without getting special permission from the FAA every time I use it. If I didn't have an experimental engine and bigger rudder, I could go with the "Standard Aerobatic" category.
If you look at airshow airplanes, check the N Numbers. If they are really big, the airplane is probably standard category. If the N Numbers are small, then it's probably either an experimental airplane that has never been certified or it is a certified airplane that is just certified in the Experimental Exhibition category.