Welcome to the forums, Cecilia.
Firstly, the combos in challenge mode aren't useless but they aren't the best combos Noel has either. Challenge mode should teach you a lot of fundamental skills required to do Noel's combos (how to chain drives strings together, 22C into dash-cancels, etc.) so it's definitely a good idea to try and get the hang of them if you can. But like you said, the combos here are different from the challenge ones since there are usually ways to get better damage from certain combos that aren't shown in challenge mode, so don't feel disheartened if you can't do a specific challenge or anything.
Second, since we don't have a completed combo list yet, I would recommend going through this video as well as this one for BnB material as well as a few more advanced combos for the time being. Of course, once the US/EU release of the game comes, we'll be focusing on optimising the combos in those videos and completing our compilation, so don't forget to check back later.
And third, if you have any more questions later on down the line, we have a Beginner Questions thread that you can post in. I'd like to think we're fairly patient around here, so feel free to ask as much as you need.
Good luck improving your game.