In the phrase, “a flock of geese,” is flock a collective noun? Is geese a common noun? Or is the common noun “a flock of geese”?

They are both common and flock is a collective noun. The whole expression is not a noun.

Nouns are single words so a flock of geese is not a noun. Flock and geese are both common nouns.

Word flock is a collective noun because it is used to describe a group of objects (here birds)