Level 18
Canonical URLs can help to better inform search engines which URLs have identical or highly similar content.
Some more information about Canonical URL's with examples https://www.shopify.ca/partners/blog/canonical-urls
They should be unique to each page and not the root domain for every page. For example if you have two pages:
- site.com/blog
- site.com/about
The canonical URL should reflect what each page's URL is. Visit the link above for some better examples