How does WhatRuns.com make money?

I use a similar site https://builtwith.com/ frequently. They have plans to get additional information.

These guys might be trying to scale until they can offer something more like this.

Also the obvious thing would be to sell data back to enterprise.

I am actually interested in what goes into building such a product. It would take tremendous resources to crawl millions of websites, not to mention fingerprinting tons of libraries and accounting for all the edge cases...

I think their TOS is fairly revealing on how they could benefit from it being free: https://www.whatruns.com/terms

See Data and Advertising