Twenty Twenty-Four Theme, where to put the personalized CSS style rules?

YES (finally)..

The specific answer for this template I found in a WP forum was a bit misleading, which took me hours lost searching for what they meant. So:

The section for personalized CSS styles is located directly in “Twenty Twenty-Four” Theme Editor, just go:

1. Appearance (themes);
2. Editor (site-editor);
3. Left Menu -> Styles (wp_global_styles);
4. Up to the left, by the Styles title, there’s an Eye and a Pencil icons, click the pencil (edit);
5. To the right, at the bottom you’ll find “Additional CSS” or at the top right, inside the three dots button.

You can also go directly to “/wp-admin/site-editor.php?path=%2Fwp_global_styles&canvas=edit” and look for step 5.

Hope this helps many newbies with this template, like me.