Where does Similar Web get visit data from?

Question

Search Engine Optimization

How does Similar Web get data (quite accurate) about visits to my site, for example?
I have GA connected and that's it. Is it that Google is leaking data to them? On what basis?
I can also get data from Google about a third-party site, or what? Or do I need to become a Jewish company like Similar Web for this?

Answer 1

malware
many many many, including just buying statistics (from mobile devices, from browser plugins), etc.
this is sometimes seen by referrals, and by the nature of statistics,
well, yes - official sources (GA and so on - also)

Answer 2

On my site, the data differs by 10 (!) times

Answer 3

if not goof look in the log