Tranco is a merge of the Alexa, Cisco Umbrella and Majestic lists. Alexa data is gathered from a browser extension [1], Cisco Umbrella is passive DNS [2]. Not totally sure about Majestic but it looks like it might be crawling of some kind, then counting links.
[1] https://kinsta.com/blog/alexa-rank/#how-is-alexa-rank-calcul...
[2] https://umbrella-static.s3-us-west-1.amazonaws.com/index.htm...