how to do diff. index.php for category and sub-category?

Wrong approach ??
You cannot have another “index” – that would break WP.
However, you should read about the Category Templates:
https://codex.www.ads-software.com/Category_Templates

Thanks for the link. After reading, I think I’ve got a way:

A customized category.php that pulls that category name into an if/then condition. If the category is a parent category, then display the loop using html template “A”.
Else, the category must be a child-category, in which case, display the loop using html template “B”.

What do you think?

I am not sure I can follow you. I am not a coder, so I’d never know how to write a code to recognize a parent or child cat.
Actually you can have N+1 category templates.

Also, be aware, Catgeory Templates are used by WP to display category archives (the posts listed when you click on a cat name) – NOT for displaying single posts.

Sorry, I should have clarified this at the beginning.
If you want the single posts to have different templates based on category, you need this plugin:
https://guff.szub.net/2005/07/21/post-templates-by-category/
(and if you feel confused by now, check out the Category Hierarchy in the Codex)

Ah, category hierarchy helps. Thanks Mo!

Hmmm… I am getting old and tired. I meant TEMPLATE hierarchy.
https://codex.www.ads-software.com/Template_Hierarchy

Yeah, I found that page when doing a search for ‘category hierarchy’.

I should have mentioned that earlier, because otherwise my above post sounds like I’m being sarcastic. Which I wasn’t.

Anyhoo, thanks again.