u/Blueberry_Unfair

I am plumbing my first pump and I'm at a loss and am not finding and videos that show just this part. What I have is a threaded part that is about 3 inches on the inlet and outlet pump. I have a threaded cumpling that fits it. A sleeve that fits in side the coupling. That all makes sense. Where I am stuck is the smooth sleeve. From what I can guess it gets glued to the end of the PVC pipe. Where in confused if it just sitting in the sleeve. Nothing holding it in no gaskets anything. So if it just sits in there what keeps it from leaking. Sorry if this is a dumb question I have tried to search and most videos are skipping this part.