I am flying SEA-ICN-DEL and am using Delta for the first leg and Asiana for the second. Would I be required to take my luggage through customs in ICN or will they transfer it through for both the let to India and to USA and I will not have to go through Korea customs?
Delta has an interline agreement for ticketing and baggage with Asiana. See: https://pro.delta.com/content/agency/gb/en/policy-library/reservations-and-ticketing/interline-electronic-ticketing-agreements--iet-.html This will cover your baggage, you need to ask at check-in to have your bags checked through to DEL.
Second get both boarding passes at check-in in SEA. They may say to you you need collect second boarding pass at Transfers gate (airside) in Korea. In which case go to the Asiana transfers counter at ICN. You do not need to clear customs/immigration to transfer At ICN.