How did the Coptic alphabet borrow letters from Greek writing?

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Thematic map of Eurasia showing the approximate geographic distribution of Indo-European language families, each represented by a different color. Areas include Celtic in western Europe, Germanic in northern and central Europe, Romance in southwestern Europe, Slavic in eastern Europe and Russia, and Indo-Iranian languages in South Asia. Non-Indo-European language regions and areas of common bilingualism are also indicated. — Greek language Approximate locations of Indo-European languages in contemporary Eurasia.

The Coptic alphabet was developed as Egyptian Christians began using the Greek alphabet to write their spoken Egyptian language during the 2nd and 3rd centuries CE.12

The Coptic writing system consists of 31 letters in total. To adapt the Greek script for this purpose, the following method was used:

Greek Borrowing: 24 letters were borrowed directly from Greek uncial writing.3
Supplementary Letters: Because the Greek alphabet lacked characters for certain Egyptian sounds, seven additional letters were incorporated from a cursive variety of Egyptian demotic writing to represent those specific phonemes.132

This adaptation allowed the Coptic language to be recorded using a modified version of the Greek script, effectively replacing earlier Egyptian writing systems like hieroglyphics.12