Re:Ask HN: Where do backlink checking services get their dat...
by sebst on Tuesday August 22, 2017 @05:06PM
(#15076393)
There is a comment which was voted dead which actually answered the question.
These services have their own crawlers. If you ever spot for example MajesticBot in your access logs you have found one of the biggest.
1) Built their own crawlers.
2) Using an Apache Nutch/Heritrix cluster in a colo facility.
3) Use 3rd party services like mixnode.