To customize a stairway for anyone is where Rhino 6's Grasshopper sub-program comes in. I guess I could have programmed one stair step tread run and riser height that would be sloped at an entered variable angle that then multiplied to become a series of steps to obtain a variable height floor to floor and extruded to the specified width.
Enter height, width and slope in Grasshopper and you would create a specific stair model in Rhino 6. Now that I say it out loud it could be fairly easy to do that.
In Rhino, like most other 3D programs, you start with a 2D drawing of a stair and extrude it into 3D. Or take a curve that twists in 3 space, place a circle at one end and extrude the circle about the curve to create a 3D pipe in three space. The possibilities are endless.