Domain authority and page authority are metrics that comes from a third party vendor (Moz) and have nothing to do with Google. Prior these metrics, Google actually had something similar called domain score and page score if I’m not mistaken, but those metrics are no longer supported. Both of these scores have a theoretical number between 1 – 100. In theory, the higher the number, the more powerful your website is considered. For example, sites like Youtube, Facebook, Google, Twitter will all have domain authorities in the high 90’s, while a new website might only have a 1.
From what I’ve read, and what I have experienced the number that influences the domain score is primarily the number of backlinks to a particular website. Sure, other things like the quality of the links or perhaps that age of the domain might be taken into consideration; however, what I do know for sure, is that the number of direct backlinks is what moves the needle for this number. With that being said, there is a belief by some of the more inexperienced S.E.Os that it will be difficult to compete against websites with high domain authority for ranking certain topics; however, this is not always the case. Currently, pages with the better relevancy are the ones that are outperforming websites competing for the same keywords even if they have a really high domain authority.
So know that we know that domain authority does not have any direct influence over S.E.O as of 2019. Is there a use at all for this metric? The metric is not at all useless, and has its benefits as a tool. For example, it can be used as an indicator of how easy or difficult it would be to rank a new webpage or make tweaks to an existing page to get it to rank better. Most importantly, the best use I’ve found is for organizing websites to approach first for my link-building efforts.