[rrk]

Either you can measure and order them of they send a guy out who does it
Yellow pine or oak treads, primed or clear stringers/risers, any style you want. If its a curved wall or an intricate design they build a wall at the shop, build the stairs, take them out and deliver and install them.

Stringers are routed, wedges in the back/under. Even cove mold is installed

 

[Railman]

It is still regional, but in this day and age, site built anything is on it's way out. One has only to look at cabinetry, & prehung doors to see the change in trends. Quality control of shop work, & shop efficiency will eventully make it universal. Anything that reduces construction time will eventually drive how it gets done.

About 10 years ago, I was asked to do rails a homeshow home about 100 miles away from Cincinnati, in Columbus Ohio. All but one of the show homes had site built stairs, that being the one I was supplying rails for. Those stairs came from a shop near mine, because no one in Columbus did them as shop built. Today, most in Columbus stairs are shop built.

 

[hdavis]

Back in the 1920s or so, a lot of cabinets were done like Ikea. Cut and flat packed and shipped by rail across the country for on site assembly / installation / finish.