Can you read files that other people make? like .ABC or whatever, I am just kind of intrested on how rom games work, mainly how the first rom was read. Or do you have to know how the file is made up, to be able to read it ?

All file types have a common storage order. Sometimes the file type is proprietary and impossible to know just how the useful data inside it is stored, but most of the time it's fairly well known.
What you're looking for is the specifications of the file format, for example, this is the specifications for the Quake 3 Map format. Every unique file type has a unique way of storing its data.

I guess I kind of missed the point of your original post, though. What I would do is try to map out the file format by looking at an open-source program that reads these ABC files. From there you could add onto that program, or make your own.
The only way I could think of that it would be possible to mirror a ROM image of a game would be to have a way to recognize the Nintendo cartridges as disks and just rip the entire disk into a raw file. I really don't know exactly how it's done though, maybe someone else here does.

Yes, you can always read files that other ppl made, but whether you can interpret the data (do something useful with it), is the question. The extension of the filename can help you to determine how the data in the file should be interpreted, but this doesn't mean that your way of interpreting the data in the file is valid.

I.e. the .BMP, .TGA and .PNG formats are well documented, but I could name my personal datafile 'NONSENSE.ZZZ' (or even 'NONSENSE.PNG' if I like breaking standards) and put data in it which only *I* can interpret.
You could read the data in the file, but you cannot interpret it the way I intended to, unless you ask me what the format of the file is, or reverse engineer my code, for that matter.

Hope that make things clearer for you :)