There's that explicit pipeline for marginalized people. However, there's long been a much more powerful implicit pipeline in the form of unconscious biases, old boys' clubs, and the like.
Yes there are exclusionary clubs which bar specific classes of minorities but on the whole they don't work that way. It's like saying frats are biased. Yes they are and the world would be better off without frats and their counterparts elsewhere, but I don't think they are mainly perpetuating racial discrimination. Rather they work as a job network and business network for their members.
I could be wrong. If so illustrate.
There was one frat in the south that was paying an old black man just to pick cotton. Seriously!
Yes, we know many of them engage in peripheral racist and misogynistic behavior, but those things are sideshows.
My point is forming these groups which foment cohesion and support for their members are rather common across cultures so to paint them as 'old boys's' networks is a misunderstanding of human nature.
Basically networks may be used to keep the other out --but that's not why or how they are formed. They are formed out of human nature to form groups which help their members --not that different from community groups, or PTAs, etc.
What do I know about medicine? If my life depended on picking a tall person hidden behind a screen, would I choose a woman or a man?
This is what's stupid about so-called "affirmative action". They've set the situation up such that it's rational to make sure your doctor is white or east Asian.