Brokers have a tough gig these days, as they do not underwrite their own loans. Instead they send it off to the bank who will buy the loan eventually. This process can be time consuming.
Big banks are bears these days. I know, I used to work at one. Closings take forever- if they can do the job at all. Without getting too technical on you, the shift in lending these days is the big banks not being able to get the job done efficiently, so others who can have stepped in. As a result, some of those big banks have laid off most of their loan officers, and concentrated on servicing loans (collecting monthly payments from borrowers after it closes). In the broker world, they still have to deal with the big bank turn times, which can be lengthy- because again they must have those banks finalise the loans.
Correspondent lenders have stepped in as that "sweet spot" for borrowers these days. These are companies who lend their own money and decision their own loans, but who will sell the loan after you close. So, you get faster closing and great rates. When the loan closes you will end up having a big bank service your loan for the rest of your loan- which, is what big banks do best these days, so that's a positive thing. Correspondent lenders use multiple sources for their loans, so they have access to many different loan types and programs and often most of the best rates just as a broker would, but without the delays.
As far as kickbacks go... it's something I have no experience in ever even seeing. People may have their preferred channels of who they choose to lend with. The lending world has become very transparent now, so any sort of fees are always displayed. Kickbacks would put a lot into jeopardy for all parties, so I can't imagine it is practiced.
Hopefully that explains a little about the market, without getting too deep.