You can tip at time of order, after an order, or both. You can also edit tip during/after order. Only the at time of order is included in estimated earnings, which I expect is the more common option since the order page sets a default tip for you. It’s more steps/effort to change tip or tip after.
I’ve heard from Uber Eats drivers about people selecting a very high tip during order to get their order prioritized or done with extras, then editing tip to a low value after.