[nunez]

I didn't build this, but here my guess: Most companies use a handful of ATS's (applicant tracking systems), like Greenhouse, Lever and Workday. Almost all of the jobs posted on these platforms are public and their top-level pages are indexable.

If I built something like this, I would start by searching for pages that have HTML fragments indicative to those systems a few times per week (since job listings don't change much).

While this won't do anything to reveal "real" ghost jobs (job reqs that are hidden or generic enough to be used for interesting referrals), it's probably a minor edge over LinkedIn Jobs (the home of stale jobs). Many of these companies cross post to those platforms anyway.