Looking for advice on a non standard approach

I'm trying to design an app/service that requires a setup that as far as I can tell from my admittedly limited understanding doesn't quite fit the standard model for multiplayer games. I'm hoping an expert can point me in the right direction in terms of unity documentation or assets to help me get started.. Just as a note in the future I would like to implement multiplayer/sharing capability so it's important I start down the right track.

What I'm looking to do is have an app that has a user authentication front end referencing a database which contains user accounts and their associated 'games' (no problem here). Every associated 'game' (which is essentially just a single Unity scene weighing 5 - 20MB) is then listed to the user who can then select and play. These 'games' (scenes) would need to be hosted on an external server or be downloaded from the server to the users app. This is the part i'm having trouble with as I don't understand if it's even possible - can a Unity app run different games that are externally hosted inside of itself? Alternatively, could in game assets be called in to a scene from an external host instead based on a user ID?

The reasoning for this particular approach is that each scene is unique/bespoke and created on a client by client basis. To create an individual app for every client would become unmanageable fast, or to package the content of all clients in to one app would increase its file size exponentially until eventually it would be too large to publish on an app store. There's no other approach I can think of..

Does anyone have some advice?

Thanks!