The short version is he finds the treasure back where he started, near an old church in Spain where he had the dream that sent him on the journey. It's the classic 'treasure was within you all along' twist, but that's selling it short. He meets a thief in the desert who mentions a recurring dream about treasure buried near an abandoned church, which Santiago realizes is his starting point. After all the trials with the crystal merchant, the Englishman, Fatima, and the alchemist, he returns, digs, and finds a chest of gold and jewels. The ending clarifies that the real treasure wasn't the physical loot, but the journey itself—the wisdom he gained, the omens he learned to read, the love he found, and his connection to the Soul of the World. It's a neat, circular conclusion that some find profoundly simple and others find overly simplistic. What happens to him after? The book leaves it open, but he plans to return to Fatima in the oasis. I like to think he uses the treasure to travel freely, maybe even becomes a kind of guide himself, having understood the Personal Legend.
A common critique is that it's a bit too tidy, but I think the emotional payoff works because Santiago's transformation feels earned. He starts as a naive shepherd and ends as someone who understands the language of the universe. The actual finding of the treasure is almost an afterthought, a final confirmation from the universe. The last scene with him promising to return to the oasis always gets me—it shows his journey integrated both his spiritual awakening and his earthly love, which is the whole point.