Hey there Wouter, thanks for posting this.
The whole tongue position issue is complex. The idea is to keep the tongue as low as it needs to be for the intended pitch. Tongue position also changes the resonance of the oral cavity and therefore the timbre of the note.
There exists the idea that the higher the tongue, the higher the note which depending on the way you talk about it could be both right and wrong…
Is the tongue higher in the upper register? YES!
Does the higher tongue itself create a higher frequency? NO!
I want the tongue to stay low in the mouth on each and every pitch but that position of “low” changes on each and every pitch.