Having Canonical Tags Set For Parameter URLs Helps Google Understand Connections Between Pages
Google tries to figure out the canonical URL for parameter pages that are included in the GSC parameter handling tool, so they may crawl these pages to identify and understand the canonical set up and connection.
Canonical Tags Should Remain The Same Between Desktop & Mobile Sites Even With Mobile First Indexing
If you have a separate m. version of your site, the canonical should remain pointing to the desktop version, despite mobile-first indexing. This is because Google will use the canonical tag to understand which of the pages belong together. Internally, Google will pick the mobile version as canonical.
Use Redirects & Canonical Tags to Stop Data From Other Site on Same IP Being Included in GSC
If there are internal links between two sites on the same IP address, data for both sites can sometimes appear in the same GSC account. If you don’t want Google crawling the other site then add redirects or canonical tags pointing to the main site.
Canonicalization is Determined on Per-page Basis not Sitewide
Google ignoring the canonical tags across an entire site because of incorrect implementation would be very rare and something that a Google engineer would need to do manually. Canonicalization is determined on a page-by-page basis and uses a number of other factors than just the canonical tag, including URL information and on-page content.
Google Takes Several Factors into Consideration When Selecting a Canonical When Multiple Variations Exist
If you have a large number of variations of one URL, for example multiple parameters, Google will take into consideration a number of factors including canonical tags, internal linking and the sitemap file, before picking a canonical version.
Hreflang Not Necessary For Non-Canonical Pages
It isn’t necessary to implement hreflang on the non-canonical version of a page because it won’t be shown in search, therefore any hreflang annotation won’t be used by Google.
Canonical Tag Can be on Either AMP Page or Legacy Page within an AMP HTML Pair
If you have the rel=amphtml tag set up correctly, Google can pick up the canonical tag from either page in the pairing.
Self-referencing Canonical Tags Are Best Practice But Not Critical
It’s best practice to have self-referencing canonical tags as the canonical tag is one of the signals that Google uses for selecting the primary page in a group of detected duplicates, however, it is not essential.
Google Will Use Other Canonicalization Factors If the Canonical Is Noindex
Google would receive conflicting signals if a canonical points to a noindex page. John suggested that Google would rely on other canonicalization factors in this scenario to decide which page should be indexed, such as internal links.