This is the viewfinder of my 40yo Konica SLR. See that red piece covering the 1.4 marking? That moves to block out any f-stops the current lens is incapable of. This is all done via physical interface with the lens. No electricity involved at all. Most of the parts that make this happen can be measured in millimeters.
At the bottom of the frame there is also a readout showing what the current shutter speed is. Again, completely mechanical. This little slide is connected to the dial by some sort of teeny tiny linkage or gear system.
And don’t even ask me how the split circle focus works cause I have no frikken idea.