There are frequently situations where silicone alone just won’t fill the gaps around the outer frame
Sometimes you’ll need to cover an old silicone line or perhaps the opening is out of square meaning there are varying width gaps around the perimeter of the outer frame where it meets the brick or block.
If the same situation arose when fitting a PVCu door or window the Installer would simply reach for the D section or 2 inch trim and the customer would rarely even know there had been a gap!
But there seems to be a definite lack of availability for aluminium trims around aluminium frames to finish off in the same manner.