Saint Mark is considered the first Pope of the Coptic church because the Coptic is related to Egypt and Saint Mark came and preached the Gospel in Egypt. It's not about tracing the church back to him. The Coptic church values Saint Paul as well as the rest disciples and Apostles of the Lord.
That is not a question that can be answered in a few sentences. One of the main differences is that Coptic priests can marry and have children. Theologically very similar. There are differences in how confession is done and in how how the liturgy is performed. Also, copts trace the church back to Saint Mark, not Saint Paul. They have different bishops and their own pope. They also operate on a different calender when it comes to fasts, feasts, and celebrations. This is a good place to start if you want to learn more. https://acofsf.com
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