This is so common, we even have ethics training at our mega tech corp and we are supposed to report any such activities. A lot of time kickbacks are subtle though like unpaid internship at vendor for CTO's niece. Or sometimes it is a bit more bold like a consulting contract that vendor need to sign up with CTO's brother's firm etc.

The most obvious and legal form of kickbacks is just regular sales process that involve a lot of nice dinners or even trips to conferences on nice resorts. One of my old boss owned his company and it was so normal for him to go out for 4 hour lunches with clients.