Where to control breadcrumbs?

Not too sure what you mean? Any examples of what you're trying to do?

Let me know :)

At the bottom of posts, there seems to be a category link, with then the next/last post in that category.

Where to control that?

Right now the next/previous links aren't controllable in the Customizer, although they will be soon.

If you let me know what you want to do to them I should be able to help out with some code :)

Okay. Right now it shows the category the post is in, followed by next/last post.

What I want is to replace that category link by a link to the sitemap. We do not use categories at all. So instead of showing a category, just show the sitemap, done.

Need to control both the url and wording for the category shown.

Will the URL and wording change depending on the post you're viewing?

No. Instead of the category name, it would say sitemap all the time. And the link would also be the same all the time to that sitemap.

All posts are kept in the same category internally in wordpress by the way. I could rename that category to sitemap and then put a redirect in place from the category to the sitemap. But it's not clean.

You could try something like this:

add_filter( 'generate_category_list_output','tu_custom_category_list' );
function tu_custom_category_list() {
	return '<a href="URL TO YOUR SITE MAP">Site map</a>';
}

Adding PHP: https://docs.generatepress.com/article/adding-php/

works. thanks

You're welcome :)