Newbie: Why using "guix edit" opens read-only files in /gnu/store?

[Rodrigo Morales]

>From what I understand, no file in `/gnu/store' can be edited by any
user. My question is: Isn't the purpose of `guix edit' edit package
definitions? Then, why when `guix edit <<package>>' is executed, a file
under /gnu/store is shown? For the record, I just executed `guix edit
emacs' and the file
/gnu/store/nxy09dxmm23krdgszgww2v80v8bkcll5-guix-module-union/share/guile/site/3.0/gnu/packages/emacs.scm
was opened.

How am I supposed to edit the package definition if it is defined in a
file under `/gnu/store' (read-only file system)?

I might have skipped an important part when reading the documentation. If
so, please point me to that part.