How to calculate an end point of a line after rotating an angle in three-dimensional space? Thanks!

Initial start position? But you said that was always just 000. Is the line also rotating around 000 (which false is assuming?)?

Or does it rotate around some other point (like the center of the line)?

Is this pure math, or are there actual gameObjects involved? With actual gameObjects, most people just make an empty child, rotate the object, then read off the child position. No math involved.