The Old Testament says nothing about Mary. Isaiah 7:14 speaks of a young woman of the time the "a young woman shall conceive" statement was made. In Hebrew it simply says that a young woman shall become/is pregnant and will give birth to a child. One need not quibble over the meaning of 'almah/bethulah in order to point out that it happens every day. Many young women become pregnant. They are virgins before (some of them) but not after. The OT says nothing about the young woman being a virgin at the time of giving birth. And of course the quote was lifted out of context and applied to Jesus. That is why Jews do not read it as having anything to do with a "virgin birth."
As for the 'almah/bethulah question, in the Septuagint Greek translation 'almah is rendered as parthenos, which can mean virgin but does not necessarily mean that. It is used in the OT of Dinah after she was raped. By the time of the Dialogue with Trypho the Jew of Justin Martyr, the meaning had apparently become more restrictive--thus the comment of Trypho that the Christians mistranslated the Isaiah 7:14 quote, as in fact they did.
As to whether the Mother of Jesus was a virgin or not, we really have no evidence at all. Both birth stories in the gospels appear to be later additions tacked onto the basic story that begins at the baptism of Jesus. Two gospels say nothing of a virgin birth. Paul says nothing of a virgin birth--in fact he speaks of Jesus being of the "seed of David" according to the flesh, meaning a descendant of David. If we accept the genealogies in the NT (which of course we should not), then Joseph is the genetic descendant of David--and of course he is supposed not to have had any physical part in the birth of Christ at all.
>From a commonsense point of view the virgin birth is nonsense.
Christianity is not alone in believing what is essentially a "remarkable
birth" motif. In Tibetan Buddhism Padmasambhava was believed to have
taken birth in a lotus blossom. Such things are metaphors, not reality.
David