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My Opencart store has an extension, which simplifies (cut names to categories from the URL structure). For example, a product page is placed under a subcategory, which resides under a main category, so the URL structure will become https://Domen.no/main category/subcategory/product, when Extension is turned off. This full URL structure will become: https://domen.no/prduct, if Extension is running. It also simplifies the URLs of subcategories: https://domen.no/subcategory, if you click on a subcategory.

I have read that the URL structure plays an important role in SEO. Now it is my question that the URLs for products can become unhealthy to SEO, when they donn't have categories in theirs URL structure? I use SEMrush as SEO tool. It reports "Fix keyword cannibalization: There are other pages on your website that are competing against the current one for the top position in search results for your target keywords." I see that some product pages are palced in the list of search results closely below theirs category page, which should be much more important than product pages. so I suspect that this URL structure whitout categories may be the reasons for this keyword cannibalization between the product pages and the category page. Perhaps this close competition situation between the pages of the same sentence leads to that Google will NOT give better rankings to the current category page. Has someone any idea?

You may get a lot of opinion on this but here's my 2cents.
* realize search engines are striving to be as human-like as possible. Search engines may tweak their algorithms but this is the direction they are heading.
* any keywords in the URL are scored highly. Putting long paths in the URL dilutes this effect
* Too much duplication dilutes the effect
* Significant related keywords boost the scoring but again, too much duplication may dilute this.

So relating this to your question, I say keep it simple and relevant, don't overuse keywords or try to use 'tricks'. Concentrate on the specifics of your category for the category, and the specifics of the product for the product and let the search engines worry about how to arrange their results.
Having category/subcategory in path should be fine in general but if you wanted to fine tune and have complete control, I would say no folder paths and repeat parts of category where required.
So rather than audio/mp3players/ipod8gb
you could have no folders and
mp3-ipod8gb
If someone searches for 'mp3 ipod 8gb' would give better product relevance than folder paths and searches for 'mp3 players' would be more likely to locate your category.