Help understanding Free at Sea 2025

Reddit and internet is getting me mixed up with the old FAS -> MAS -> 2025 FAS stuff. I need help understanding the current 2025 Free at Sea / beverage gratuities vs Ship gratuities.

New to NCL with a recently booked cruise via Costco. I wanted to add Free At Sea for the Unlimited Open Bar. The operator was telling me that FAS is unlimited alcohol but I would still be charged gratuities at the end of my cruise, and was trying to convince me to pre-pay gratuities.

I thought FAS come with all gratuities paid? So I can order drinks and not worry about a huge gratuity bill at the end, right? Unless the operator was mistaking my question about gratuities to mean ship gratuities. My understanding is that those ship gratuities can either be pre-paid before boarding or the same amount will be billed at the end of my cruise. So whether paying now or at end, I would be responsible for paying ship gratuities?

TL;DR question: what exactly does Free at Sea cover, and will I be responsible for any extra alcohol costs if I purchased FAS? Extra question: how does ship gratuities work, and will I still be charged the exact same ship gratuities if I don't pre-pay them?