Template override for Category Blog with Subcategory title as headline between Articles

I am trying to create a category blog layout which looks like this:

Subcategory 1 Title

Subcategory 1 Article  1
Subcategory 1 Article  2
Subcategory 1 Article  3

Subcategory 2 Title

Subcategory 2 Article  1
...

1. I am unsure if this is an override of "article", "category", or "categories".
2. If I put an override in the templates/shaperhelixultimate/overrides/com_content/ folder, I cannot select it in the menu item layout options. I only find overrides from the folder /shaper_helixultimate/html/com_content/article

Can someone help me which file I need to edit to create that override and where to put it?