Overriding Files from third party module

Question

I'm new to Magento 2 and still trying to get my head around the folder hierarchy.

If I want a file from my theme to replace a file in a module I am doing the following.

1. For css, original file that I want to override is here.

   vendor\supplier\module-search-autocomplete\src\SearchAutocomplete\view\frontend\web\css\source\module_cssfile.less

I have copied it to the below location for edit.

app\design\frontend\themesupplier\my_theme\module-search-autocomplete\src\SearchAutocomplete\view\frontend\web\css\source\module\_cssfile.less

2. php file.

Original is:-

src\vendor\supplier\module-search-autocomplete\src\SearchAutocomplete\Index\Magento\Catalog\Product.php

Copied to:-

app\design\frontend\themesupplier\my_theme\module-search-autocomplete\src\SearchAutocomplete\Index\Magento\Catalog\Product.php

Where did I go wrong ?

I am flushing the cache and also have tried client side compile mode, but changes do not show up.

Clearly I have got the pathing wrong for the copied file, but where ?