This is the end, my friend
As you reach the end of Tears of the Kingdom, you’ll get the main quest to “Find Ganondorf.” Although you can continue with the other main quests through this point and eventually uncover where Ganondorf is hiding: you can also just…go there. Here’s a full walkthrough of how to find him.
Ganondorf is hiding below Hyrule Castle in the depths
Although I did all the legwork here, you can just reap the benefits!
Ganondorf is at the end of a semi-large series of tunnels below Hyrule Castle, at the “root” below the depths. A very easy way to get down there is to shoot out of Lookout Landing Skyview Tower, then glide on over and dive into the chasm. Continue down the pit, and when the game text shows “Gloom’s Approach,” you’re in the right area. It was right under our noses the entire time!
All you need to do is look for an opening, signified by a Like Like (slug-like thing). Go down that tunnel until you reach the end of the path, where Ganondorf is located. Before you actually do battle with him you’ll need to take on a series of miniboss fights, then you can’t hard save immediately afterward: so make sure you’re ready to go when you drop down the final time (the game will ask if you want to “dive” before you do that at the end of a path). Thankfully, falling in battle with Ganondorf will result in loading a shadow auto-save right before the fight (but you can’t teleport out).
Note that you can put down a Traveler’s Medallion right in front of the final dive to the last boss fight. If you don’t have a Traveler’s Medallion yet, start the Robbie questline at Lookout Landing!