Does a transit visa need to be obtained when flying from the third country to China with a layover in South Korea?

Don’t need it if not leaving from the airport. You can spend the night at the airport.
No need, no need to go through customs if not exiting the airport.

It doesn’t matter if you’re not leaving the airport, after all. You can spend the night at the airport.
No need, no visa required if not departing from airport.