I'm sure many of us are wondering the same thing here, so thought I'd bring it up:

To what extent are we allowed to use existing assets in our project?

For example, are we allowed to use Creative Commons-licensed images and sound created by other people in our game? What about prefabs or scripts from the Unity Asset store (free or paid)? Can we re-use assets from other projects we've made ourselves?

And if we do use existing assets, should we cite our sources for these somewhere in our project?

Thanks for clarifying.

You're fine to use third-party assets in your project, though they will need to be licensed/attributed appropriately, either in-game or in documentation as required.

This is pretty clear-cut for visual, audio and other non-code assets, but can get a bit murkier with external code and scripts. As you're being assessed on your coding ability, you need to make it clear what you've used in addition to your own work. If it becomes difficult for an assessor to identify what you've actually done, then this may cause an issue.

Exellent; thanks for making that clear.