Getting stuck editing templates

Thank you so much for your beautiful themes! I use Koji when building sites with my students and it is PERFECT. I am building a site now with Vermeer (my first experience with full site editing). I’m using the site to build a database of reading experiences (I’m used ACF to make a field group linked to custom taxonomies and visible in a custom post type). I want to reveal what is in the database on the front end through archive pages for terms in the custom taxonomies. I don’t want to link to posts or view content of these posts because there is no content. My first thought was to create term-specific templates (one for each term in each custom taxonomy–something like 80 total). I was able to create a pattern that I inserted into the template (replacing the default blocks pulling in title, image, excerpt). I’m happy with how that displays on a computer, but the grid doesn’t show up on mobile. I’ve waited many hours and tried multiple browsers and devices, so I don’t think it’s a cache issue. I’m worried I might have removed something in the template that accommodates for mobile.

As I realized this snag, I also realized that the standard index included with the theme is automatically creating archive pages for specific terms within my custom taxonomies (hooray! how did I not realize this would happen automatically?). This will save me a ton of time, but I’d still like to customize the index. When I try to edit the index, though, I can’t seem to make changes within the post template. When I look at template parts, I’m also not able to edit the loop. (I have an image of this that I can share via link if this isn’t clear).

Should I be able to edit the default index page so my changes apply to every archive page? I’m wondering if perhaps it’s deliberately locked so I don’t inadvertently break my site?

Thank you for any guidance you might have. I’m loving this theme and really want to make this work!