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Rel canonical is used to avoid duplicate content issues, on pages that display the same content under different URLs.

In your case the use of rel canonical is not appropriate.

Best practice, if you want all content to be assigned to the main URL, is to actually put all content on this page. You should get rid of these product/main-page/specs, product/main-page/resources, etc pages and post all the content on product/main-page using divs.

Google doesn't believe these links are natural ones because they represent no value for a website visitor. From what I know the advice is to make them nofollow if you still want to keep it in the footer of the websites.

Personalty I would go with another solution - I would place a dofollow link on a single page like for example the "About" page if it contains details about the website also and not only the company or on an "Impressum" page if the website has one.