The Landing of Elendil

[Voralphion]

I am not sure about the situation in Arnor, but there were already many numenoreans living in the area that became gondor at the downfall of numenor.

Long before in the days of their power the mariners of Numenor established a haven and strong places about the mouths of the Anduin, in despite of Sauron in the Black Land that lay nigh upon the east. In later days to this haven came only the Faithful of Numenor, and many therfore of the folk of the coastlands in that region were in whole or part akin to the Elf-Friends and the people of Elendil, and they welcomed his sons.

Of The Rings of Power and The Third Age

The ships of Isildur and Anarion didn't carry all the people of Gondor, there were many there already who welcomed Elendil's sons as their kings. This was probably the same for Arnor, with some of the Faithful who had previously left Numenor living in the north near Gil Galad.
Elendil and his sons didn't completely establish new realms, they joined and took the rule of existing realms.