Help with theme files

I am using the softshell theme.

It has ‘duplicated’ files like

\wp-content\themes\softshell\pluggable.php

that I THINK are being used.

These files are also in

\wp-includes\pluggable.php

(in this instance, they are the same file content, but I want to
makes some changes to this file’s content — whichever one is being used)

Are the theme files overriding the regular files?

I didn’t think “pluggable.php” was one of the “theme-able” files.

Does it just use the theme file if it is there and it overrides the existing normally-used file, or always, or what’s going on?

TIA