This is a tough area. It depends on the playing level you are on. That’s why you’ve gotten incomplete answers.
(Tongue level only is NOT true. They use the embouchure to make register changes like low to high. So the normal description you get from this group is a very inaccurate one trying to show an idea by leaving out the other factors like chops and air,)
(The reason that air alone doesn’t work well until they are more advanced is because they don’t have the embouchure muscles or control to handle that technique. They blat rather than play music. This ruins any concept of tone that they are trying to develop. That’s why I teach it last.)
In advanced players I take the work away from the lips and make them learn to use AIR. (Think gut slurs.)
In intermediate students I focus on the tongue arch.
With beginners they need to learn to move the embouchure.
When I play lip slurs in music I will use all 3 things embouchure, tongue arch and air. If the slurs are big enough I will even use a pivot. This is why an experienced player can do the Irons so much easier and quicker.