How to Create a CMS Collection List with User Forms

Hi all. I have a client wanting to have a website with a CMS collection list and a form connected to it so user can firstly see all of his items available and then fill out a form of what they want. It needs to be connected so when they add an item to the CMS it also gets added in the form and users also need to be able to select (add) quantity per item. Can anyone explain me (or show me where I can learn) how to achieve this? Thanks in advance.