You’d think this question would have a simple answer, right? I’ll give a somewhat over-generalized answer. For more elaborate biryanis at parties, it is often served as you described. However, for this presentation, the biryani has to be fresh out of the oven. Otherwise, it’s hard to mix it in your plate once it’s cold. For everyday eating, based on what I’ve witnessed, it’s mixed in advance. Your eaters’ arrival schedules are staggered, plus it’s easier for kids to eat.
Personally, I will always pre-mix because many of my guests aren’t used to the layered presentation. Often they will take too much of the rice layer, or too much of the meat layer. And even if the ratio is correct, they won’t mix it in their plates. They’ll end up eating the rice with something else and then the meat layer is too strong on its own, so it gets tossed. One of my Aunties actually leaves the layers intact in half the serving tray, and mixes the other half. That way people can serve themselves from the mixed half, but still see the presentation.
