Short answers, pulled from the story.
The Eastern Orthodox Church, officially the Orthodox Catholic Church, is a communion of autocephalous national and regional Eastern Christian churches. As of 2020 it has approximately 220 million adherents, making it the second-largest Christian body in the world after the Catholic Church.
The Eastern Orthodox Church has no central authority comparable to the pope. The Ecumenical Patriarch of Constantinople is recognised as primus inter pares, first among equals, but he holds no real authority over churches other than the Constantinopolitan church. Each autocephalous church is governed by its own bishops through local synods.
The split, known as the East-West Schism, is traditionally dated to 1054, though it was a gradual process rather than a sudden break. Doctrinal issues such as the filioque clause and the authority of the pope were worsened by political, cultural, and linguistic differences between Latins and Greeks. The breach deepened after the Fourth Crusade sacked Constantinople in 1204.
The seven ecumenical councils are Nicaea I in 325, Constantinople I in 381, Ephesus in 431, Chalcedon in 451, Constantinople II in 553, Constantinople III in 681, and Nicaea II in 787. In Orthodox teaching an ecumenical council is the supreme authority that can resolve a contested matter of faith.
From ancient times through the first millennium, Greek was the shared language across the regions where the Byzantine Empire flourished and the language of the New Testament. After 1054, the label Greek Orthodox marked a church as being in communion with Constantinople. Today only a minority of Eastern Orthodox adherents use Greek in worship.
On the 11th of October 2018 the Ecumenical Patriarchate of Constantinople revoked the document that had let the Russian Church ordain the Metropolitan of Kyiv, and on the 5th of January 2019 Bartholomew I granted autocephaly to the Orthodox Church of Ukraine. In response the Russian Orthodox Church severed all ties with Constantinople, and the two have not been in full communion since.
The Eastern Orthodox Church is the primary religious confession in Russia, Ukraine, Romania, Greece, Belarus, Serbia, Bulgaria, Georgia, Moldova, North Macedonia, Cyprus, and Montenegro. Roughly half of all Eastern Orthodox Christians live in Russia.