This page of the Horizon Forbidden West guide provides the location of the Tallnecks available in The Shining Wastes. This is a special activity that reveals a sizable portion of the map. However, it is worth remembering that it only shows the terrain and does not unlock individual activities. This means they'll fill your map with question marks you'll have to discover on your own.
None. It's part of the quest. This Tallneck doesn't count towards side activities, despite appearing on the map.
You'll notice this Tallneck in the first few hours of gameplay. However, you won't be able to climb it. On the map, it will be marked with a little padlock, indicating that you are currently unable to override this machine.
It will become available during The Wings of the Ten main quest. We have described it on a separate page of our guide.
Overriding the Sunwing
This quest involves overriding a flying animal. You'll be able to move about the map in ways previously unknown to Aloy. By flying, she will be able to reach formerly inaccessible locations.
Overriding the Tallneck
Your task is to fly towards the aforementioned machine. It's an unskippable mission objective.
This is the easiest Tallneck in the entire game. All you have to do is fly up to it and land on its head. Do this by pressing the special button (R2 for PS). All that's left to do to finish the quest is to override the machine.