Inverting the position between post title and thumbnail-image

Hello,

You’ll need to edit your theme archive template. Of course, having a child theme is mandatory to not lose your customization on next theme update.

Because how the templates are made is different from 1 theme to another, we would need at least the theme name to inspect it.

I’m using a starter theme: underscore…

But I would invert the position also in the posts pages, not only in archive pages

OK, all the pages or only an archive, the job is the same. you need to dig in the templates to modify them.

You could copy archive.php, renaming it category.php. Edit the category.php to get a different, custom template part that outputs in the desired order. The template part can be based on a renamed copy of /template-parts/content.php

Or just edit archive.php to conditionally get a different template part based on is_category().

Or reorder the elements with CSS flex box model. Include the body tag’s “category” class as a selector to only affect category requests.