What is the possessive form of the word auditorium in a sentence that describes the curtains that belong to the auditorium thus the auditoriums curtains?

The possessive form of the Word auditorium in a sentence that describes the curtains that belong to the auditorium, would be the auditorium's (singular possessive). If you wanted to describe 3 auditoriums with the same curtains, you would write auditoriums' (apostrophe s for plural possessive).

The auditorium's curtains were gold with black trim.

The auditorium's curtains were gold with black trim in New York City, but two other auditoriums' curtains in Atlanta, George were black with gold trim.

I study auditoriums' curtains as a hobby, but I have yet to find one auditorium's curtains have polka dots.