On this page of the Far Cry 6 guide you'll find a detailed description of the treasure hunt Oku's Triada Relic, available in the Valle De Oro region.
Subregion: Cruz Del Salvador.
District: Arroz Flatlands.
First things first, we should mention this quest can only be completed at night. You won't be able to access the building during the day.
The task is relatively simple and linear. You need to reach the treasure and the shrine. First, you're going to need a key. You have to take several steps to find one. First off, break the mirror by the door. You'll find a switch which will open the next room.
In said room you'll encounter a short puzzle. You must press the buttons in the correct order, as shown on the picture above. First the middle one, then the one on the left, finally the one on the right.
Go upstairs. There, read a note on the piano and wait for the passageway leading to the key to open.
It is time to go down to the basement, which you can open with the key.
You need to break the planks to reveal the passageway. You should use the machete to do so, guns would create unnecessary noise.
When you enter the next room go to its very end and look left. You'll spot another red button you have to press.
It's time to jump down into the well, where the passage opened.
You can collect the treasure there, but the quest itself is completed by collecting the relic on top of the shrine.