An "Aperature" is ONLY the lips. It changes in size based on how high/low and loud/soft we play. That being said, the lips do not create a "set" sized hole. The lips flap to open and close the hole. It is purely the strength and agility of the muscles controlling the embouchure vs the breath control that lets us get good. NO AMOUNT OF READING OR ADVICE WILL HELP YOUR CHOPS. MONTHS AND YEARS OF TRAINING ARE REQUIRED TO GET EVERYTHING IN SYNC! Check this out: IWK Brass Research As far as the teeth go, they are a bit of support for the lips when playing. When the teeth are further apart, the geometry of the face changes. That is why we should not need to "think about this". With enough practice (that you need anyway), the face will gravitate to a favorable position.