On Thu, Nov 26, 2020 at 06:22:26PM +0100, Christopher Dimech wrote:

[...]

> Does that mean that setq-default also enables automatic filling?
> 
> I need some explanation about setq, setq-default, and set-local.

You should skim the manual's "Variables" section [1]

In a nutshell, variables are a way to attach a value to a
name. In Emacs, there are different kinds of variables.
For our current purposes, we consider global variables
and buffer-local variables (there are more!).

A global variable is seen from everywhere. If you set its
value while in buffer A, this change will be seen in every
other buffer.

Buffer-local variables can have one value per-buffer. It
shadows the global variable (if there's any with the same
name). Most mode things are buffer local. The `fill-column'
is one example. You might want different values for it
in different buffers (say, you're editing two sources with
different styles in one session), that's why it is set up
as a buffer-local variable.

So if you do (setq fill-column 72) in the context of buffer
A, only buffer A gets to see that.

Setq-local (not set-local, as you wrote) is just a convenience,
which makes the named variables buffer-local and sets their
values (thus leaving possibly like-named global variables
alone).

The setq-default is for setting one or more buffer-local
variables's default values.

The gory details are in the Elisp manual [3]. There's also
an explanation for setq-default which is much better than
I could come up with here.

Cheers

[1] Online here:
    For the user:
    https://www.gnu.org/software/emacs/manual/html_node/emacs/Variables.html#Variables
    For the lisp programmer:
    https://www.gnu.org/software/emacs/manual/html_node/elisp/Variables.html#Variables

    although you /should/ have those manuals with your Emacs installation.
    This is preferrable, since they should correspond to your Emacs
    version.

[2] https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer_002dLocal-Variables.html#Buffer_002dLocal-Variables