Beating the final boss in chapter 14 unlocks the “Homecoming” trophy / achievement. After the ending you will be asked if you want to create a save game. Choose yes and you can load this save later to free roam after the story. You can go back everywhere in the open world to finish up side quests and other activities you have not yet completed (nothing is missable). At a lodging you can use Umbra’s powers to return to Lucis & Altissia.
The final boss is “Ardyn”. A character you will encounter numerous times throughout the story. He uses the same weapon attacks as you. He switches between different weapon types but the combos are the same you are used to from your own weapons.
Phase his attacks (hold Square-Button) and attack after he finishes a combo. Counter-attack whenever possible. If you’re lucky you will get the L2/LT button prompt to summon a god which will instantly deplete his health. In the second part of the fight you are both flying around. When you hold down the attack button it will automatically fire projectiles at him. He deals a lot of damage in this part so use health potions if necessary.
After that you just need to finish a little QTE event and enjoy the ending cutscene.