Our data comes from 4 main sources: 1) A panel of over 200 million monitored devices, currently the largest panel in the industry. 2) Local internet service providers (ISPs) located in many different countries. 3) Our web crawlers that scan every public website to create a highly accurate map of the digital world, and 4) Hundreds of thousands of direct measurement sources from websites and apps that are connected with us directly. This last source of data helps us to constantly improve our learning set, fine tune our algorithms and reach accurate estimations about traffic stats for ALL websites and mobile apps.

mehr infos:
https://www.similarweb.com/downloads...ethodology.pdf

wenn die natürlich daten von ISPs bekommen, macht es die sache um einiges einfacher für sie