What exactly are the files . and .. on mac? Anecdotally, files called . and .. seem to often (always?) be present inside otherwise 'empty' directories on macOS.
Example
Create a new directory (with nothing in it)
mkdir test

cd into it
cd test

and list its contents
ls -a
.   ..

What exactly are these files . and ..?
Technically and practically, they represent directory entries for the current directory (.) and the parent directory (..), respectively.
So cd . always keeps you in the current directory (useful mainly in scripts when a directory is assigned to a variable and the value should represent just the current directory), whereas cd .. always takes you one level up (unless you are already at /, in which case .. is the same as .).