Right now the norm is to combine wastes, add it to drinking water, and then spend a lot of money to get the water out of it again.
But yeah, scaling it up massively is the trick: https://www.oursoil.org/food-tank-soil-transform-human-waste...
Most dairy farms are very efficient at collecting all the waste the cows produce. They then empty the pits into a tanker truck and spray/inject it onto the fields. It goes a long way and helps complete the cycle, but your intuition is right, it alone is not enough.
Some of the nitrogen in your waste won't make it back to the plants. There are other micro nutrients that plants need that won't make it back either.
https://www.milorganite.com/what-is-milorganite
So they use the sewage from the existing treatment system, let microbes process it all, and then extract/dry those microbes.
I was more thinking about needing means to rethink our toilet/sewage system entirely with a focus on initial separation.