Google's dominance over the search engine industry that governs what is effectively a critical fraction of all online culture leads to it being both a clear and obvious trendsetter and an entity capable of determining and enforcing rules that all businesses must follow. Companies everywhere invest considerable resources into placing in the highest positions of relevant SERPs generated for queries based on commonly understood terms and phrases, and Google must both keep its ranking algorithms hidden and regularly update them to combat efforts to abuse its systems.
One of a countless number of Google-inspired trends that have entrenched themselves within online culture is the distinction between external links that point to other websites with the intent to help their search rankings and external links that do nothing more than provide the navigation. By default, an outgoing link helps its target's image in the perspective of Google because Google is allowed to consider that link to be reflective of interest in the website that has not been artificially generated. Websites that exist to aggregate links for other websites in large quantities, on the other hand, are among those that elect to attach the "nofollow" attribute to each link so that whatever "link credit" it potentially had to offer does not count. Getting backlinks without this attribute to appear on external websites without using black-hat tactics is a discipline in its own right.
Google's more recent changes to its procedures has resulted in two new attributes that websites' owners can attach to their outgoing links so that Google can exercise a more nuanced perspective when deciding whether to avoid dispensing link credit for a given link. The first is the "sponsored' attribute, which informs Google that the link was created out of some sort of commercial contract involving advertising and the like. The second is the "ugc" attribute, which is meant to denote backlinks that are hosted within websites' comment sections and other areas where users submit content not necessarily authored by the webmasters. The full scope of the implications of these elements for backlink culture is not yet entirely understood. For more information click here https://www.reddit.com/r/SEO/comments/d2acdd/googleevolvesnofollowandintroducestwonew/.