restrict access to categories/products by shopper group or Joomla Group

[codingmall]

This 3rd party extension can do what you are looking for:
https://codingmall.com/products-mainmenu-8/169-joomla-access-control-for-front-end

and here is a link explaining the implementation:
https://codingmall.com/faq/7-joomla-products/168-joomla-frontend-access-control