if a rangefinder (for example Bessa R2A) has 35/50/75/90 frame lines, what happens if you put on a 15mm lens or 21mm lens? how do you frame your shots? which lens (35/50/75/90) should you set it too even tho none of them fits the lens?

Auxiliary viewfinder that mounts in the hotshoe is used for framing, you still use the rangefinder window for focus.

The lens will trigger a set of framelines, usually the 28/90 lines if it's wider than what is available. Leica have been sneaky and some of the newer lenses trigger different framelines to competitors lenses in equivalent focal lengths, to avoid a simple coding job to emulate the Leica corrections.

what if it's an older non-digital rangefinder where you have to manually set the frame lines?

Sexymaru wrote:
what if it's an older non-digital rangefinder where you have to manually set the frame lines?

Framelines are for framing, you dont use the viewfinder for framing only for focusing. Just ignore them.

In practice with a wide lens you hardly ever use the VF at all, you scale/zone focus using the numbers on the lens.