I'm a high school teacher, so my experience isn't directly relevant but I've certainly encountered my fair share of cheating.

The student that wrote the code can always explain not only how it works but why they made certain decisions along the way.

The student clueless enough to copy someone else's code usually can't even explain its function, much less explain tradeoffs.

If they need to present it on Monday to the team you can basically exclude that. If they do that they can't understand it well enough to really present it.