Jack, I think the answers would depend on which area of the hobby/industry you're interested in.
At present, there are about two dozen major airgun manufacturers, and a slew of small ones. Breaking into that end would be rather costly. The largest US based manufacturer is Crosman/Benjamin, and overall they 're also one of the biggest world wide. They make arrangers, nitro piston, and PCP airguns in just about every configuration you can find.
Most of the smaller ones, like Wicked Air Rifles, is owned by Jim Gaska. He also owns a CNC machine shop. In his case, it is sort of an extension of his hobby, and he's managed to make a few dollars along the way. There are quite a few garage-shop guys, making high-quality, but low production airguns. Most of the smaller shops are all making PCPs. I personally don't know of any making springers and pumps.
The are at least 50 guys out there doing tuning, doing mods, and selling parts for factory guns, primarily PCPs. There are yet another bunch making specialty parts for specific models, like air stripers. One of the most popular "parts" are upgraded suppressors, aka silencers. How many are making those world wide is anyone's guess, but it is a big number!
Sub-facets within this hobby are as numerous as one's imagination. Some are really into FT (field target) meets, while others are just tinkerers. And Lord knows there are the collectors, some of which literally have hundreds of airguns.
Personally, I love to hunt, and I have a unique situation where in I can hunt almost every day of the year if I so choose. I ended up owning this site, but it wasn't intended—I just fell into it. Perhaps you will find your nitch too.