By now is similar to already, but it is used in a different sense. Already tends to be used in the present or past perfect. By now emphasises the time that something has taken.
He should have finished by now (already)— Debería haber terminado ya (By now emphasises the time taken)
Many people are, by now (already), used to it— Muchas personas ya están acostumbradas (By now emphasises that it has taken time)
They would have arrived by now (already) if they were coming — Ya habrían llegado (a estas alturas) si fueran a venir (By now emphasises that they have taken longer than expected)