You are not wrong. WP provides nowhere to edit content for the main taxonomy archive page = archive-{cpt-slug}.php
The only way to accomplish this is to add your own admin page, like an ACF options page.
Nor is there a place in the DB set aside to store this information, it must be stored in the options table.
Or as an alternative as a “Page” with a custom page template.
archive-{cpt-slug}.php could be used to load this custom page template, or the page template could be used to load archive-{cpt-slug}.php via get_template_part().
