The chapter below contains a complete walkthrough for The Womb of the Mountain, including screenshots and boldfaced tips.
After the unfortunate event during the Proving, Aloy wakes up inside the sacred mountain. Follow the only available path - your equipment will be waiting for you along the way. Open the main door by interacting with it and proceed forward. Turn left behind the second door to find your weapons and the rest of your equipment. Follow Teersa and see how the events unfold.
We are back in the village. To regain access to the wilds, go talk to the new leader of warriors. Before you do so, try to pick-up all the plants and chests in the area, as their contents may prove to be very useful. In a short while you'll be repelling an attack - prepare as many arrows as you can. Stock up on fire arrows, as they are the most useful projectile from the lot.
The tribe will be attacked by a Corrupter, a very agile machine with lots of durability. This fight can be tough, especially that there is a group of Striders running around the battlefield. You can ignore them and direct your whole attention to the more dangerous enemy. If you brush against any of the Striders, try to hit them with your spear - 1 or 2 strikes should be enough to eliminate a typical unit.
Corrupter's weak spot is his head. Keep him in your sights and focus on aiming. The remaining members of the tribe will help you, but you'll be the one that has to eliminate the Corrupter.
Your enemy will leave an interesting device that enables you to take control over certain robots. Take a look at the chapter dedicated to machines to learn more about the device itself. The current quest will end, giving way to a new one - The Seeker at the Gate.