When I was working at AT&T, there was a different kind of kickback scheme at play.
AT&T --> Primary Vendor (B) --> Another Vendor (C) --> H1B/others who work as contractors for C.
Hiring managers and C have some sort of arrangement, wherein managers get paid in cash by C. Primary Vendor (B) takes a cut for $3 per hour per candidate. Even (B) knows what is going on, but does not care as long as it is easy money.
Anyway, one such scheme got busted when I left that place. AT&T fired three employees over this--two of these employees are brothers. The gist of it was that (C) was controlled by wives of two of these employees. And that (C) was placing 90% of candidates in AT&T NJ. This had raised suspicions--and other primary vendors complained about this. So, AT&T found the conflict of interest--that (C) was controlled by wives. And one of the wives was working as a contractor for one of the brothers. Because of this, they terminated all three.
Another company I am aware of, is doing this in the bay area. Here, the primary vendor is itself controlled by a dad of one of the managers at company D. This son at D pays in cash to the prospective managers, when their candidate is picked up.
One company I worked at, had a different kind of play: primary vendor's hot recruiter was involved with one of the hiring managers. Eventually, this led to the termination of that said manager.
Lots of things go on behind the scenes.