A Way to look at the game's files? E.g. unused content, models, sprites

[MajinBuu21]

I see so many hackers opening up N64 roms and other games and finding beta content like SM64 had. People wondered what the shadow people of hell valley's texture in Super Mario Galaxy 2 was called. A hacker acessed the game and found it to be "Helly Valley Sky trees". How did they do that? How do you open up the game and look at the unused content and graphics?

 

[Fanatic 64]

Suffice to say it wildly differs from game to game (and even more from platform to platform), and requires advanced technical knowledge. Mainly with hexadecimal editors and memory viewers, etc.