Why don't you see binary code when you open a binary file with text editor?

Binary and text data aren't separated: They are simply data. It depends on the interpretation that makes them one or the other. If you open binary data (such as an image file) in a text editor, much of it won't make sense, because it does not fit your chosen interpretation (as text). What you call text is a subset of the possible file contents: Data that in a given character set translates to readable characters. For example, in ASCII, you can see that, of 128 "allowed" values, only about half …