Extend Magento Luma theme less

When extending Magento Luma less files in one children theme, what exactly is the name and the complete structure of the “parent” folder (mentioned at https://developer.adobe.com/commerce/frontend-core/guide/css/quickstart/customize-styles/#extend-parent-styles) ?

I find the documentation very fuzzy here and I need help with this. I only can assume at this point we don’t want to add the new file _extend-child.less on core parent theme folder.

Thank you very much in advance!
See below the diagram from the documentation link.

app/design/frontend/Vendor/
├── parent
│   └── web
│       └── css
│           └── source
│               ├── _extend-child.less (keep this file empty)
│               └── _extend.less
└── child
    └── web
        └── css
            └── source
                └── _extend-child.less
...