2-3. About 3000mah phone batteries are common but these are different voltage hence different Watt-hour. As a crude yardstick use 2/3rds the division of mAh. E.g 10000/3000/3*2= 2.2
You have typical 3.7V inside the Powercore, it is stepped up to 5V hence the email@example.comV becomes 7400@5V, then the phone steps down to 4.2V, there are losses in the steps up down and the sockets and cable usually losing 20%. So a simple division of mAh is not valid. Division of Wh is better but you struggle to know that.
I often see negative Amazon feedback due to this lack of knowledge and it's not Anker design fault but due to everyone quoting mAh when Wh should be our unit.
This explains it deeper: