Hi,
Your questions have nothing to do with this thread. Please open a new thread for unrelated questions.
In the link www mysite/products_list_a/category/products_list_b, products_list_a is the alias of the menu item, "category" is the "Category SEF name" of the HikaShop configuration (so that HikaShop can differentiate between a product page and a listing of products of a category) and "products_list_a" is the alias of the category to use as parent instead of the category selected in the "main category" setting of the menu item (it is used to display sub categories (and their sub categories, etc) of the main category).
As you said, in this link, products_list_b can be replaced by the alias of any category.
In each category, you should fill the "canonical URL" field with the main URL you want google to use to index the category.
When that setting is filled, regardless of the URL google uses to access the category, it will only index that canonical URL and won't penalize you for having other URLs to access the same page.
And it's the same for the "canonical URL" field of the products.
HikaShop also has the "force canonical URLs on listings" setting which can potentially automatically generate the "canonical URLs", however the URL selected (among all the possible ones) might not be the one you want.