An old game speaks the language of an old console, and the Mac doesn’t know that language. We translate the whole game once, like a book — and the Mac reads it on its own.
Why is the game built on your machine? Because the installer has no piece of anyone else’s game in it — just a recipe. The groceries are yours, we only tell you what to chop.
For nerds. This is static recompilation. The game’s machine code was written for an old processor, and we translate all of it into the language of Apple chips ahead of time, not piece by piece while you play. Graphics that called Direct3D get rewritten for Metal. Then Apple’s compiler (those same “developer tools”) builds a native app out of it. While you play, there is no translator between the game and the Mac.