Thanks for the question.
When exporting a file from Fusion (or any CAD application) we look at the origin and coordinate system of that file - so were your XY, XZ and YZ planes are. See picture below…
So ideally you would have your part/assembly aligned around these planes in some way - often this would either be with all the planes going through the centre (good design intent for mirroring etc.) or treating one of the planes as the “floor” so its sat on it.
You can rotate the entire assembly by pressing A to go into assembly selection mode, then click anywhere in the assembly, it should highlight blue and you will see the movement triad, press ALT to change to rotate and then you can drag one of the rings to rotate.