There were many prophets. Matthew was speaking of Isaiah. AT: "the prophet Isaiah" (See: [[rc://en/ta/man/translate/figs-explicit]])
# Behold ... Immanuel
Here Matthew quotes the prophet Isaiah.
# Behold
"Look" or "Listen" or "Pay attention to what I am about to tell you." This word adds emphasis to what is said next.
# Immanuel
This is a male name. (See: [[rc://en/ta/man/translate/translate-names]])
# which means, "God with us."
This is not in the book of Isaiah. Matthew is explaining the meaning of the name "Immanuel." You could translate it as a separate sentence. AT: "This name means 'God with us.'"