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I searched online. It seems very rare people discuss how TBA is valued. Not sure if I could get some high level understanding on this.My current understanding is that since it is essentially a forward on MBS pool. We could use MC to value the MBS pool to the settlement date and then convert it to the NPV value at as of dates. Correct?Any guidance on paper or short answers are appreciated.

Take a look in Veronesi's book. I agree with you that it's difficult to find information on how price it however it is a bit more complicated than you think. On top of the fixed income tools you need data on defaults and prepayments.