For most woodworkers, I'd say the analogy is closest to making "plugins" for existing tools. They require a little knowledge (which easily obtained on YouTube), and can greatly increase your ability to use the base tool.

One example is a table saw sled, which helps you make clean, repeatable, 90º cuts. It's one of the most useful "plugins" you can have for your table saw, and relatively easy to make. Making the actual table saw, on the other hand, would be expensive and very difficult, and would require knowledge of many fields (metal working, electrical systems, etc.).

For a lot of custom carpentry, it's pretty common for carpenters to make their own jigs to solve specific problems (e.g., if you need to use your router a bit like a thickness plane to mill down some object by a couple of millimetres along some axis so it fits somewhere, you probably want to build a jig first).

Pretty common, I think. I've made clamps and rove punches for instance, and I've been tempted to make drill bits. (But this is for wooden boatbuilding, which is its own uniquely crotchety corner of carpentry. Not unlike emacs...)

Did you write Emacs? Did I write Emacs?