First, the Hebrew word עַלְמָה (almah) which is translated "virgin" in Isa 7:14 actually only means "damsel/maiden" - a woman of marriageable age who may or may not be a virgin depending on whether she is already married or not. It occurs in only seven places: Gen 24:43, Ex 2:8, Ps 68:25, Prov 30:19, SS 1:3, 6:8, Isa 7:14. [Another Hebrew word, "Bethulah" specifically describes a virgin.]
Isa 7:14 is a prophecy about a child that Isaiah's wife would have as a sign that Assyria would be defeated. It is a local prophecy about a local situation and carries no immediate Messianic overtones. This can be seen in Isa 8 when Abi, the wife of Isaiah conceived (V3) and bore a son. This was declared to be the "sign" in V18.
Here am I, and the children the LORD has given me as signs and
symbols in Israel from the LORD of Hosts, who dwells on Mount Zion.
Thus, Isaiah's prophecy was fulfilled when his wife conceived and gave him a son.
However, the inspired NT writer, Matthew, in ch 1:23 takes this prophecy and re-purposes it to apply to Christ. This could not have been deduced from the original text but by the direction of the Holy Spirit it now has become a Messianic prophecy about Jesus.
Further, in quoting Isa 7:14, Matthew specifically uses the Greek word for "virgin", namely, παρθένος (parthenos), thus making this a significant Messianic, second (dual) prophecy.
The Pulpit commentary observes this:
May not the true account of the matter be that suggested by Bishop
Lowth - that the prophecy had a double bearing and a double
fulfillment? "The obvious and literal meaning of the prophecy is
this," he says: "that within the time that a young woman, now a
virgin, should conceive and bring forth a child, and that child should
arrive at such an age as to distinguish between good and evil, that
is, within a few years, the enemies of Judah should be destroyed." But
the prophecy was so worded, he adds, as to have a further meaning,
which wan even "the original design and principal intention of the
prophet," viz. the Messianic one. All the expressions of the prophecy
do not suit both its intentions - some are selected with reference to
the first, others with reference to the second fulfillment - but all
suit one or the other, and some suit both. The first child may have
received the name Immanuel (comp. Ittiel) from a faithful Jewish
mother, who believed that God was with his people, whatever dangers
threatened, and may have reached years of discretion about the time
that Samaria was carried away captive. The second child is the true
"Immanuel," "God with us," the king of Isaiah 8:8; it is his mother
who is pointed at in the expression, "the virgin," and on his account
is the grand preamble; through him the people of God, the true Israel,
is delivered from its spiritual enemies, sin and Satan - two kings who
continually threaten it.