Duplicating a List of Things

Let me start with an apology because I’ve seen this referenced so much, but none of the solutions are presented very clearly. Ok here’s my structure.

I’m building a services marketplace where brand can create a campaign and then buy services. The campaigns have milestones and when you purchase a service I want to duplicate those milestones and then add the new milestones to the package purchased.

I’ve added a list, but that doesn’t duplicate the milestones so as services are delivered on the milestone they change the original campaign’s milestone status, which doesn’t work since you can have multiple packages assigned to a single campaign.

I tried copy list of Campaign Milestones to Packages and that just gave me nothing. I’ve started reading about backend workflows and APIs but not super clear on how to implement so if that’s the solution a video or a a really basic explanation would be appreciated.