So im new to the dr.browns bottles so i change the nipple for my bby but im i supposed to also buy…

So im new to the dr.browns bottles so i change the nipple for my bby but im i supposed to also buy the white thing ? Since i put the nipple and it wont stay with the white thing it has im not sure whats it called but its the one tht olds the nipple thing.. i hope yall understand me lol buy anyways im i supposed to buy that thing too ? Or only the nipple since it wont fit.