Nevermind, fixed it already. For anyone wondering the same thing, I used the “Smooth” modifier and ticked the “Auto Smooth” box.
I’m a bit hasty at asking for help - well, whatever.
The original question:
Is there any particular reason why e.g. extruded faces aren’t smoothed anymore, even though the original primitive was?
Here, I have this cylinder:
Then I extrude from it:
Why is it that the new faces aren’t smoothed anymore?