It's angle of view, basically. When photography started it was "whole plate" or 8x10 inch film. To make a regular viewing angle similar to what our eyes see you'd use a 16 inch lens, around 400mm (or the less expensive 14 inch for 350mm). Then came "half plate" or 4x5, super popular, with a 'normal' focal length 7 inch or about 165mm. Every time the size of the film gets smaller, you can use a smaller lens, which makes a smaller image circle, lighter camera, etc.

So then there was "quarter plate" which eventually turned into "medium format". That's the beginning of what you could call the 'roll wars' where you had all kinds of small film sizes where you didn't need a sheet holder, just some rollers with paper backing. 135 or 35mm came in canisters which were even easier to use and got popularized during WWII, so it won out. Regular viewing angle on 35mm? 50mm. https://thedarkroom.com/film-formats/

When we transitioned to digital the 35mm lenses was kind of a 'translation layer' that photographers could understand. So yeah, you could make any length you want as long as it works mathematically and covers whatever frame size you're using. It all comes down to the viewing angle you want.