With the Nemesis eliminated, your mission is complete, and extraction is your final objective. However, the area will still be swarming with infected and possibly remaining hostile forces. The path to the extraction point will be relatively straightforward but not without danger. Expect a final push from various enemy types, including a few lingering Tyrants or G-Adults.
Conserve any remaining health and ammo. Your team will likely be low on resources after the Nemesis fight. Stick together and move cautiously. The extraction point will be a designated area, usually a rooftop or an open street, where a helicopter will pick you up. Defend the extraction zone from incoming enemies until the helicopter arrives and you can board it. This marks the end of the mission and prepares you for the final confrontation.