How do most people meet their future spouse?

There's no single answer to this; but a good summation might be through a shared activity. A shared activity would include work, a mutual hobby, or a preferred hang-out spot; you first have to meet someone in order to start dating them.

Second place, especially these days, is likely to be via online dating and matching sites; also possible would be through the intervention of mutual friends (set-ups and blind dates).