Okay, how does this work realistically? What about people that are sterile? Or people who choose to not have children due to their genetic history? Or what if t

Okay, how does this work realistically? What about people that are sterile? Or people who choose to not have children due to their genetic history? Or what if they literally cannot afford to have children? And what does sustainable mean in this context? Is every nation supposed to have a positive growth of population into perpetuity?