The information on this is very confidential between the casino and the 3rd party service provider, so you will not get "clear" answers on that. Logic says that they enter into a contractual agreement to take a percentage of the profits made on those Slots and that they have some kind of "hire" agreement in place... with options to change the RTP.

It is a myth that the RTP and Variance are being changed on the fly, but we know different casinos can run the same Slot on different RTP, so you will have to do some research to see which Slot is running what RTP to improve your odds. (Note it will be 1 or 2 percent at best, so it does not make a world of difference)