One must understand that the breakup of the Ottoman Empire, shortly after the end of the First World War, led to a political reconfiguration of greater Anatolia.
Much of present-day Turkey exists within the millennia old land of Anatolia-(then renamed, "Asia Minor" during The Roman Empire 2000 years ago). The contemporary country of Turkey exists, primarily due to the efforts and workings of Kemal Ataturk.
However, shortly before the rise of Kemal Ataturk, Anatolian Turkey, was essentially divided into various European territorial zones of occupation......this included, a Greek occupational zone, which returned the region of Ionia and in particular the city of Smyrna/(Izmir) to Greece in the year, 1919. However, the Hellenic controlled zone of Ionia and Smyrna only lasted 3 years and by 1922, the Greek population of Smyrna were literally chased out by Kemal Ataturk and the rabidly nationalistic Turkish military. The region of Ionia and he city of Smyrna/(Izmir), in particular, had been "ethnically cleansed" of its centuries old Greek population and the limited sovereignty that the Greeks had over Ionia and Smyrna-(as guaranteed by The Treaty of Sevres)....was discontinued.
Beginning in 1922-23, the Republic of Turkey was born whereby much of Turkey's present-day borders have remained largely unchanged in 100 years. Though it should also be noted that around the same time when the Republic of Turkey was founded, there was a population exchange between Greece and Turkey whereby hundreds of thousands-(or perhaps even millions) of Turkish Muslims living in Greece-(primarily, Northern Greece), were relocated to and exchanged with millions of Anatolian Greeks-(residing primarily in the Black Sea region) who were subsequently, relocated to Northern Greece.
As for present-day Istanbul, it is a city of about 6 million people, the vast majority of its residents are Turkish Muslims. The Greek Christian population-(which is primarily associated with the Orthodox Christian Patriarchate), numbers in the thousands. Judging from its demographics, the Greek Christian presence in present-day Istanbul, is nearly infinitesimal in size. In other words, the heyday of Byzantine Constantinople, is a historical memory whereby the impressively designed Dome of the Saint Sophia Cathedral....remains surrounded and punctuated by four minarets....and has remained so, since 1453.