Where does SimilarWeb truly get its majority of data from?

They claim they get their data from contributory network (most websites haven't connected their analytics to similar web), partnership like ISP (honestly, which ISP is selling this data, and even if they sell, you would at least need to buy data from a majority of ISPs in the world) and public data extraction (which I believe that data isn't reliable).

What I doubt is most people use the web from their mobile phones, and mobile phones browsers don't have extension …

analytics claim data isp isps network partnership privacy sell selling similarweb web websites world