How do they do so much without a 3rd trimester ultrasound?

Me

I was told my 20week anatomy scan was my last ultrasound unless something was wrong, but it seems so unsafe to not have one towards the end of pregnancy.

How do they tell the baby’s weight? Positioning? How do they know if the cord is around their neck? Or if there is low fluid levels? Or a slew of other things?

Literally, do they just assume all those things are fine? Do you not get to know the baby’s weight estimate without an ultrasound?