I have just learned that B5 and B6 are simple confirmations. B1 to B 4 could be used for emphasis.
The simple past version of the conversation in post #1 is then:
A1. Were there many old people in the crowd?
B1. No, there weren't. (There were mostly young people.)
This time there is not an auxiliary verb in that question.
So, if we want to make a simple confirmation, we just say yes, or 'yes + a pronoun + an auxiliary verb.'
When the verb "be" does not have an auxiliary, we repeat the appropriate form of "be."