In immersive mode

• Swipe up on the right part of the screen to climb when you're near an obstacle
• Swipe down on the right part of the screen to crouch

In standard mode

• Click on the jump button to climb when you're near an obstacle
• Click on the crouch button to crouch

You can also choose to climb and crouch automaticaly in Settings > Accessibility. Beware, crouching only works automatically when you need to bend down to pass under a rock or other element of the landscape. To hide in the grass, you will need to perform the action yourself.