So do you believe the second person of the trinity was conceived in Mary?
This is why I ask: “Is ‘that holy thing which shall be born of’ Mary and ‘shall be called the Son of God’ the same God that is the second person of the Trinity.
Because if the answer is yes, then Gabriel told Mary that she conceived the person who in John 1 was called the Word who was in the beginning, was with God, was God; who was the Word made flesh and dwelt amongst us; who was Jesus Christ; who was the only begotten Son in the bosom of the Father. It was also the person John the Baptist called the Lamb of God, Andrew said to Simon was the Messiah, Philip said to Nathanael was the one of whom Moses and the prophets wrote, Jesus of Nazareth, and who Nathanael called the Son of God, the King of Israel.
If the answer is no, who is that holy thing that is born of Mary?
Again I ask, can you direct me to any teachings that expand on how Mary is the mother of Jesus and not the mother of God?
That is a deceptive question. Originally you have used the word "conceived". Now you drop the word "conceived" and switch your approach. Mary did not "conceive" (bring into being) the second person of the trinity. Yet you have consistently tried to get me to agree that Mary conceived the second person of the trinity. She did not. Therefore she can not be considered the "mother" of God. She can, as the Holy Spirit inspired the writers to write be considered the mother of Jesus because she did conceive (bring into being) the man Jesus. I'll simply leave you now to the torment of trying to figure that out.