How does Similarweb calculate website traffic? Does Google Analytics sell them some data? As far as I know, it's against the rules but I wouldn't be surprised if Google Analytics sells them because they are shady. I think that the only other way for Similarweb to collect data is if website owners connect their Google Analytics to Similarweb apps but I doubt top websites do that because it's not a good idea to share their data for nothing in exchange.
I think that another source of traffic estimation for Similarweb will be through adware extensions, browsers, softwares and VPNs but how accurate is that? I can't imagine 26 million people visiting Freebitco for gambling when there are so many issues around them. I also can't imagine two men taking care of the website with such a huge traffic.
From what i read online, they collect data from various sources, but most of what they listed looks SUS. However one method that could actually be the source of data are web browser extensions, which collect data and distribute it publicly.
Either way, their report seems to be far from perfect and i wouldn't rely too much on it
It is also possible they collect such data through connecting to the API of those sites, this is the most possible way I could think of right now even though I actually don't have any idea of how it all works or will work, but I only can say this because I've seen some sites monitor traffic from another site by simply connecting to the site through the site's API key.
We can draw example from coinmarketcap, coingecko and other market price aggregation platforms, how do they get the price of every crypto coin on the exchanges and average it? It's through the exchanges APIs, API in a lame man's definition is a way to mirror what is going on or happening on another website and display it on your own website.
API can also be the way through which similarweb monitor and collet traffic data from all the websites around the world and show it on their own site.