Why big ecommerce site SEO has optimized their internal searcing result pages? LIKE TARGET; Amazon etc...

Recently I've been researching big e-commerce sites' on-page optimization and I found many sites using search result pages for indexation and are on top of google ranks; such as target; amazon; aliexpress; Has anyone done it before, using on site search result pages for indexation? If allow google to index some on-site search result pages, with proper selection, would it be good overall speaking for SEO? for me I think it boots some keywords ranking as you can have tons of keywords and relevant products on that pages. Like you can have unlimited category pages. For instance, If you have a category page for apples, then you could have additional pages for "red apples“ ”green apples" "crispy apples"...etc by using the search result? instead of making a lot of category pages that might look messy on navigation place?

Search result pages, if indexed, will help with internal links to products.
I guess it's the way for them to do internal linking.