The "closed call" belonging to the zero initial is directly at the beginning of the vowel U. Because they have no initial, they need a "sound-proof symbol". According to the hanyu pinyin scheme, writing begins with the letter w.
In actual pronunciation, the lips are closer than the pure vowel uu, and there will be slight friction when passing (that is, the lips need to be brought together into a small "circle" and the base of the tongue is slightly "raised" backwards).
The initial consonant of Putonghua has only one labial sound of "upper teeth biting lower lip": F.
These theories will not be mentioned in the Putonghua test, as long as you pronounce them correctly!