For asking a query, is it required to be logged in?
No. The user has to enter his details after tapping the ask a question option on the product page and then tap the submit button.
Does the admin respond to the queries or the sellers?
The admin and sellers can respond to the queries as per the configuration set by the admin.
Does the admin have the option of entering a custom email for receiving the query?
Yes. The admin can enter a custom email address that is used for receiving all customer queries for all the products.
What is the configuration that allows the sellers to receive queries for their products?
The field for the custom email needs to be set as empty, and then the sellers will receive the query on their email address automatically.