i'll expand a little bit for the broader group seeing as we're here
so i used SimilarWeb to see how many monthly visitors a site had and occasionally to confirm that the country where almost everyone is coming from matches my expectation based on the content on or linking to the site. if the number of visitors is small, similarweb does not have enough data, in which case you can use the alexa rank to see if it's is below 300,000 or so which tells you the traffic is above 50,000 or so. for visited sites you will see the number of visits with similarweb, along with the mix at the bottom, which gives you the breakdown of search, social, referral, display, and email for desktop visitors, so you can see on which medium/platform a site's content/marketing focus is and if the overall site is healthy in terms of visitor numbers over time. for example, a site with a high average number of visitors over the last 6 months with most of their visitors coming from social and with visitor numbers that spike up and down month-to-month is buying media at an arguably effective enough rate to decide to keep allocating spend but lacking stable campaigns. this is common. if the visitor numbers for that social driven site were flat, or increasing at a significant rate, you can see that they are returning more than they put in and that the campaigns are healthy. you want to double check this, they might be diversifying channels and pulling levers enough on the quality of the visitors to keep numbers flat. either way, they have something working as long as it's not just them burning capital. this is where doing a little research in the terms and conditions to find the company and using google, linkedin, and facebook can help you understand their background.
alternatively, for small numbers of visitors it could be organic social, you can get a breakdown of the referring domains for social traffic in the similarweb web app. you might see a link to the facebook post that is sending them all of their visitors. Or, a Youtube link, however if the content is not directly related it was a Youtube ad. If similarweb says it's display that is driving their visitor numbers you could put their domain into ahrefs and look at the backlinks they are getting to get an idea of who they have formed their best collaborations with. if it's email, join their email list.
for many of the sites of people reading this they are almost entirely search and have flat or slightly increasing visitor numbers. obviously this is ideal but even still if you're anything like me once you see enough examples of hastily thrown-together upstarts riding the wave of an underexplored paid channel you start opening your eyes to new opportunities to grow.
alexa... alexa showed me similar sites which had an audience overlap with the site I took interest in. sometimes it's a mixture of friends/competitors. for media buying-driven sites it may include one or more of their partners' pages. it's ranked so that helps. if you have a search-driven site, you might take a look at one of these media-buying driven sites in alexa to get an idea of what might work for promotable services/products to marry with your organic content. for either type of site the site flow is helpful at the bottom showing you where people go after they are on that site. if the site is a product page, you'll want to see where folks came from to learn who else is doing it well. that's how I found the site in brazil I mentioned.
similarweb, alexa, ahrefs rely at least partially on imperfect clickstream data that is provided by website owners or browser extensions and anonymized for our consumption via these tools. not nearly imperfect enough to not use :). there's no substitute for writing content that helps people but for knowing what to write about these tools might be useful. hope that helps