Accessing files inside roms?

I see all these videos of people accessing beta content in games, and I've always wanted to see that for myself, so I was wondering what the specific programs and methods they went about doing this were? And would I go about doing them?

depends entirely on the rom and the system its for.

beta and the complete release of a game are seperate, yet similar. beta games are released to playtesters, who play the game to make sure no error, mispelling, typos, bad grammar, fatal crashes, and all those nasty stuff gamers dont want to see. After that the official release of the game is given to the general public.

What I mean by beta is the unused content in a game that's locked away in the files.

oh thats just junk code. best way to find them is with a hex program, and other romhacking tools. these are from programmers who either didnt use them, or just got lazy and didnt remove them. most times it just items that dont exist, or unused sprites, nothing significant to the actual game.