I worked as a waiter when I was a student. Most of us did. The underlined word is a/an-

As as a preposition:
We use as with a noun to refer to the role or purpose of a person or thing.
I worked as a waiter when I was a student. Most of us did.
We don’t use as + noun to mean ‘similar to’. We use like + noun:
It’s almost like a real beach, but it’s actually artificial.