For those looking to find more, this is generally about the translation of the Hebrew ʿalmah (עלמה) to the context-sensitive Greek parthénos (παρθένος) in the Septuagint, the Greek Old Testament. The passage that gets most attention is Isaiah 7:14 as it seems to have been interpreted by the gospel writers as a prophecy of Jesus (or a suitable prophecy to model the gospels on):
Therefore the Lord himself will give you a sign. Look, the young woman is with child and shall bear a son, and shall name him Immanuel
For those looking to find more, this is generally about the translation of the Hebrew ʿalmah (עלמה) to the context-sensitive Greek parthénos (παρθένος) in the Septuagint, the Greek Old Testament. The passage that gets most attention is Isaiah 7:14 as it seems to have been interpreted by the gospel writers as a prophecy of Jesus (or a suitable prophecy to model the gospels on):