Andrij. I pronounce you Complete and Utter GIT
It doesn’t matter where you start. Just start.
Kim, you are very bad!
Rear derailleurs don't have two jockey wheels .... the have one, the top one. The lower one is actually called a tension wheel.
The thing in the middle of a bottom bracket is not an axle; it's a spindle because it spins. The thing in the middle of a hub is an axle because it doesn't spin, but something spins around it.
An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle is supported. In the latter case, a bearing or bushing sits inside the hole in the wheel to allow the wheel or gear to rotate around the axle. Sometimes, especially on bicycles, the latter type is referred to as a spindle.
And in Railway applications, the f-off huge lump of round bar that joins one wheel to the other, both mechanically and electrically, is never ever ever going to be called a "spindle."