Vim and newlines

Questions:

• Does Vim use NUL to represent newlines, or vice versa?
• Why does Vim represent NUL and newlines the same way sometimes? Is it just some historical thing, or is there still a valid technical reason for doing this?
• What do \n, \r, ^J, and ^@ mean in different contexts? For instance \n means something different on each side of :substitute.
• Why does Vim append ^J to the end of a register ending in ^M? For instance, try :let @a='a^M' then :reg a, where ^M is inserted by doing <C-V><C-M>.
• How does this newline representation come into play with the different line endings used on Windows, Mac, and Unix?

Some references to check out:

• “Newlines and nulls in Vim script”
• Various help pages