Isaiah 7:14 (NNRV) 14. Therefore the Lord himself will give…

Isaiah 7:14 (NNRV)
14. Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive, and bear a son, and shall call his name Immanuel.
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Matthew 1:21 (NNRV)
21. And she shall bring forth a son, and thou shalt call his name JESUS: for he shall save his people from their sins.
Here I would like to ask a question. I would be grateful if I could get the right answer. Immanuel = God with us. Jesus = Savior.But if someone asked me why the Lord’s name would be Emmanuel by Isaiah who came to me, why was Joseph asked to keep Jesus? How would I explain it to him?