Muslim population numbers are deceiving. There might be 9% of population but you should look at how it is distributed between ages. Muslim families might have ten children and local families a single child or childless. In Germany it is normal that schools in ‘non-Muslim’ areas made by immigrant kids by half. In more immigrant areas there might be one or two white kids per class of twenty five pupils.
It is not hard to imagine how it all would look a generation later.
We in Los Angeles have a pretty good visual of how that works out. Not by birth but by border crossing.