On 4 Sep 2025, at 07:31, Eli Zaretskii <eliz@gnu.org> wrote:

(SLNX-files not added to any auto-mode-list, because I couldn't
find where/if that is done in the nxml-mode codebase).

It's in files.el, where auto-mode-list is set up:

   ("\\.x[ms]l\\'" . xml-mode)

and "C-h f xml-mode RET" says

 xml-mode is an alias for ‘nxml-mode’ in ‘nxml-mode.el’.