What measurements the window builder has to make, i.e. where exactly to make the measurements, depends, of course, on the level at which the windows will be installed.
Regardless of the exact installation level: the fact that measurements have to be taken at all is related to the tolerances between the planned dimensions and the reality of execution. In this sense, you can only measure what is already there. So far, this applies only to the structural envelope of the wall and the cladding of the wall.
But that is enough, which is simply due to the order of ranking: compared to a window, the wall ranks higher – it is better to know the final size of the window dressing later than to have to hammer out a few more bricks – whether masonry or facing bricks. However, compared to ETICS, the window has a higher rank: the slabs are slightly cut to a different size (and in any case are only brought to the exact size during installation, where they are the last slabs in their row). So here the window doesn't have to follow the "wall" and may already be there. It should be discussed with the contractors whether it should already be installed.