They are not "supposed to" but face it, in the past they were "not supposed to" pay players under the table but we know it was done.
It basically works like this...you have a dude at the NIL collective who is "friends" with the staff, the staff says "We have this guy and he wants to be a Husker, so make sure it happens" and then the NIL guys try to make it happen.
That is assuming there is a good relationship between your NIL dude and the staff.