Shower plumbing

Hi all, just joined. I had the house reolumbed and going to put in shower base - poly marble that has 50 mm outlet built into the base. The plumbers have 100 mm pipe coming up to floor. The guy said to put in puddle flange in his way out. Once that is recessed and waterproofed I will have a 100 mm pipe at the surface, does the shower outlet just sit in there or do I need to when the base is out on mortar or does it need to be connected in some way, what way? I was planning to do as I had done for repairs in the past, cut it back and join the pipes with socket connectors. Don't know what is the go now