Alexa is my go-to place to get a first impression on how much traffic a website gets.
I have never looked into how they get their data, I always assumed they get it from internet providers?
Is there an alternative? SimilarWeb publishes their data with a huge delay, so it is not of much use for me. From my experience, it is also less reliable.
It is crazy how valuable internet properties get closed down when they are owned by giant companies. According to Alexa, Alexa is still a top-5000 site:
https://www.alexa.com/siteinfo/alexa.com
It seems insane to just close it down.