It’s possible to obtain estimated website traffic data through different tools, such as SimilarWeb or Alexa, which all have different degrees of features and accuracy levels.
There are also other tools that are designed for specific purposes, all of which use educated methodologies in order to estimate traffic. Remember that this is only estimated, and not true traffic stats, which would be much more accurate.
There are ways you can use trending and comparison data, and not just top-line traffic, so long as you’re careful how you use competition and competitive sites.
Search Engine Land has written a post where they look at 6 different ways you can use website traffic estimation tools in order to provide specific SEO insights beyond regular traffic.