When you land, head to the left into the cave and when prompted you can start 'sprinting' (really slow for a sprinter eh?) and go all the way to the end of the cave. Interact with the red board in the water for your Water Unu, then head back out of the cave. Head down the path to your left all the way to the end and interact with the lever mechanism on the right, beside the lever. Turn around and follow the path to a little open spot on the left, where you can use the brass key acquired earlier to open a lockbox with a picture of the stone key location. Backtrack down the main path and look for another path to left where you can see a rock with a flower crown on it, and a flower crown on the ground. Just next to the crown on the ground is a rock you can interact with to reveal the stone key needed to progress forward. Head back to the gate and interact with the lever mechanism on the right, hit both levers to move forward through the gate.
Move forward and you will come across a camp where you can interact with the photo of expedition crew on a box behind the bonfire spot. Keep moving down the path until you find a red board with a hole on the left side of the path and interact with it to find your Mountain Unu. Keep following the path next to the red board and eventually you'll spot another red board slightly submerged in the water that you can interact with to find your Fish Unu. Keep moving forward and shortly you'll see a path go to the right with some planks on the ground, take that path and go forward until you find another red board to interact with which will be your Sun Unu. Backtrack to the previous area and this time we'll take the left path where we'll come to an area with a whole bunch of red boards. Walk straight ahead and up the rocks and look to the right below, where you should see a barrel to interact with, giving you an achievement. Pictures are provided below in spoilers.
Climb down from the rocks and in the middle of the area is a board with symbols that look like ">>>" with a birdhouse on it. That is the only board in the area you can interact with for your Bird Unu. If you are facing the barrel for the achievement, head to the right where you can interact with a lever to open the shortcut back to the starting area of the chapter. Head back to the camp with the bonfire, go up the path to the left and interact with the puzzle pillar. All the Unu's you have collected will be inputted into the pillar by moving the symbols and it's just by logically what would make sense, so from top to bottom you would go Sun->Bird->Mountain->Water->Fish.
Hit the red button and the bridge will form, but before crossing return back to the starting area where you landed and interact with the marks in the sand. It seems someone has made off with your speedboat!
Head back and cross the bridge, now follow the linear path to complete the chapter, unlocking:
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