How are doughnuts filled? I thought they were just injected in but that would compress the dough.
The custard jam etc does not appear to be cooked so I assume it is filled after the cooking process.
But the filling does not share space with the dough. (It is in place of rather than being mixed together).
The dough does not seem to be compressed around where the filling is added.
There are no joining marks where the dough could have been removed and replaced with the filling.