North Macedonia and the Republic of North Macedonija are two different terms.
The Republic of North Macedonija it is a Former Yugoslav Republic that became an independent country on 1991 after Yugoslavia break apart and it is located North of Macedonia. Prior it was called Vardarska Banovina, with a parallel name of Socialistic Republic of Macedonija after 1944 as region of Yugoslavia with no means of a republic as a counttry, but as an administrative region, after the communist Yugoslavia followed the Soviet scheme and turn the regions into socialistic republics in planning for expansion under the communist political agenda. It is also called North Macedonija as per Prespa Agreement, however has little to do with Macedonia and North Macedonia. The Republic of North Macedonija is occupying the North Macedonia region — the area of Monastir — on its south region. The ancient name of the region that now is been occupied by the Republic of North Macedonija, it was called Peonia, however this modern state has no connection either with the ancient Peonia or Macedonia, as its population arrived in the region only after the 6th century AD along with other slavic and Bulgarian migrations.
North Macedonia is the region of Monastir, traditionally a Greek territory with Greek population. During the centuries and because of the Ottoman period settlements the composition of population change and many of the native Macedonians move south to safer areas. Before and during the Balkan wars, Greece didn't have the time to liberate the area as Serbians advance over the Bulgarians, while in Western, Central and East Macedonia, the Macedonian Fighters and the Greek Army advance over Turks. It was a strategic decision for the Macedonians either to move East or North. The political decision was to advance East and liberate the North afterwards. Other political events as the Balkan wars didn't make it possible to move North at the time. North Macedonia is an area of Macedonia that it is outside the Greek border and today it is a part of a newly made country that has no historical connection to ancient Macedonia.