Depending on what time you wish to eat, you can also book some speciality restaurants (not all are additional costs) before embarkation. This tends to be pre 6:45pm ish when the virtual queuing begins.
We have just returned and the app is very simple, outside the main restaurants there is also a line to just queue for those not wanting to use the app. We also noted some restaurants (glass house / olive grove) using pagers for those that had just dropped in. As you would expect peak times are busier.
there is also just the horizon buffet which doesn’t need booking.