Your phone/os generate raw inputs (eg touch events in the format of: posx, posy, pressure). Each app then make use of these raw inputs in its own way. SwiftKey

Your phone/os generate raw inputs (eg touch events in the format of: posx, posy, pressure). Each app then make use of these raw inputs in its own way. SwiftKey will give you some words. Piano app will pay a sound. And So on. Interpreting and converting these raw inputs into what a user wants, is literally what an app gets paid for.