There's no set formula for a link indexing service, but they're mainly using a combination of backlinks on already indexed webpages (including Google News sites), and manipulation of various Google services (url inspection, cloud, slides, faked chrome browsing data, adsense approval submission, etc...). At least these are the methods I've managed to reverse engineer.
From the tools you mentioned, GSA and Scrapebox build links on statistic websites (which in turn need to be indexed nowadays, so completely useless), and Omega used to set up 301 redirects to the urls to index, and request indexing for those 301s via GSC - though some people have reported they now build links on spammy pages.