Short answers, pulled from the story.
By 381, these churches collectively formed the state church of the Roman Empire. The Oriental Orthodox Churches trace their lineage directly back to ancient foundations established across the Middle East and North Africa during the first century of the Common Era.
Hierarchs leading what would become Oriental Orthodoxy rejected this formulation entirely because they believed it implied Nestorianism which separated Christ into two distinct persons. Cyril of Alexandria had previously advocated for the formula stating there is one incarnate nature of God the Word instead of the dyophysitism declared by the council.
Umayyad rulers taxed Christians at higher rates than Muslims and these tax policies drove merchants toward Islam and undermined the economic base of the Coptic Church. Jerry Bentley noted in his 1993 work that these measures made it difficult for the church to retain Egyptian elites under Arab-Islamic governance.
In 1959, the Ethiopian Orthodox Tewahedo Church received autocephaly from Coptic Pope Cyril VI. This marked a significant step toward national independence for African churches before the Eritrean Church gained full autocephaly in 1993 following political independence from Ethiopia.
A common statement emerged in 1984 between Syriac Patriarch Mar Ignatius Zakka I Iwas and Roman Pope John Paul II. Another agreement followed in 1990 between the Malankara Orthodox Syrians and Romans while Armenians and Romans signed a declaration in 1996.
Approximately 71 million members belong to the Oriental Orthodox Churches according to 2020 estimates. The majority of adherents live in Egypt, Ethiopia, Eritrea, India, Syria, Turkey, and Armenia with the Ethiopian Church remaining the largest of all Oriental Orthodox churches with around 36 million members.