Correct way to change bytes in a file?

[Ofnuts]

You don't have to worry about how things are stored. There is an immutable one and a mutable one. They could be stored exactly the same way, with just an "immutable" flag somewhere. Or one is stored as a group of contiguous bytes (or ints), while the other is a bunch of individual byte objects (control block+byte/int value). The immutable one can be used in sets and as a dictionary key because it is hashable. Not the other. The immutable one is iterable and indexable, but that doesn't make it a list, even if the lists are iterable and indexable.

A very informative video on the subject: https://www.youtube.com/watch?v=_AEJHKGk9ns