How do cross-console games work, codewise?

When a company decides to make a new game cross-console, what happens in terms of the code for the game? I'm thinking of major games i.e. Bioshock, Call of Duty, etc. that run on xbox, ps3, wii and/or PC.

When a game is being designed, does it have to be designed with each platform's API (assuming they all have different APIs)? I just picture it in my head as game companies doing three times the work for three consoles, but I know that must not be rig开发者_开发知识库ht. How do they get essentially the same game on different consoles?

It's all about abstraction, never call the os/console's file api or gfx api directly, abstract it. If you do that you will only have to change your abstractions, and not find every trace of the other os/console in your program, and change it.