src: employee-rights lawyer buddies who have sued companies over this.
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.
Turns out the technical consultants were making $15 an hour with no per diem or benefits and their functional people were getting about $22. The company I worked for got blamed for the living conditions of the employees... but we weren't in charge of their living conditions in the first place... that was all handled by their direct employer.