[Luk 1:34-35 ESV] (34) And Mary said to the angel, "How will this be, since I am a virgin?" (35) And the angel answered her, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy--the Son of God.
According to the Bible:
“What is conceived in her is from the Holy Spirit” (Matthew 1:20).
The Virgin Mary “was found to be with child through the Holy Spirit” (Matthew 1:18).
“God sent His Son, born of a woman” (Galatians 4:4).
That’s what Protestants believe transpired. We don’t speculate; we just believe what the Bible says. If the Bible doesn’t provide any additional information, then we don’t need to know. God, through the Holy Spirit, was responsible for Mary becoming pregnant. However, I did find an article on the subject that expresses the Protestant view, part of which says this:
From these passages, it is certainly clear that Jesus’ birth was the result of the Holy Spirit working within Mary’s body. The immaterial (the Spirit) and the material (Mary’s womb) were both involved. Mary, of course, could not impregnate herself, and in that sense she was simply a “vessel.” Only God could perform the miracle of the Incarnation.
However, denying a physical connection between Mary and Jesus would imply that Jesus was not truly human. Scripture teaches that Jesus was fully human, with a physical body like ours. This He received from Mary. At the same time, Jesus was fully God, with an eternal, sinless nature (John 1:14; 1 Timothy 3:16; Hebrews 2:14-17.)
Jesus was not born in sin; that is, He had no sin nature (Hebrews 7:26). It would seem that the sin nature is passed down from generation to generation through the father (Romans 5:12, 17, 19). The Virgin Birth circumvented the transmission of the sin nature and allowed the eternal God to become a perfect man. Source: https://www.gotquestions.org/virgin-birth.html
Jesus, fully human (through being born of a woman) and fully God (through the Holy Spirit).
No doubt you will receive more learned and scholastic answers, but this is the best I could come up with before I shut down my laptop and serve up our evening meal.