99 Altima Exhaust manifold failure P0420

Hello sir,
My 99 Altima GLE @77600 miles has "Service engine soon" light on, the code is P0420. My dealer replaced Oxygen sensor twice but the light would go off then but will come back again after a couple of days.
Then a Firestone mechanic found "Exhaust manifod/Catalytic converter" failure. He says its actually catalytic cnvrt. That has failed, but need to replace the whole exhaust manifold assembly.

Iam confused now, difference b/w Catalytic converter and exhaust manifold. Also a website says exhaust parts like Catalytic converter is covered by Nissan for 8 years/80,000 miles. Does this mean exhaust manifold is not covered. Can you please clarify.