Me, I learned that if you want to create a cave, you can just pick a stone model, size it up, and put the player inside.

I learned about baking light maps

That Android can't open local files with default permissions

The difference between Forward / Mobile / Compatibility Renderers 

and how to make a VR Game 😁

I learned the audio bus system, recording, playback, etc.

I also learned that next time I should test it on a fresh device without an existing shader cache...

I learned about physics process priority. Unfortunately, I learned after the deadline and the bug (hands lagging behind) is in the submitted version. :(

• If I don't use Godot in a few months, even after making multiple games with it, I forget almost everything, especially the XR stuff
• Godot is really easy and quick to relearn.
• Working with rotations via quaternions is still a pain with Godot, especially with scaled objects. Maybe I have to use just direction vectors and look_at to rotate something without messing with its scale or dealing with the Vector3 rotation angles?
• For rigidbodies, if you want to directly set its linear velocity and rotation every frame, you need to use _integrate_forces in your code, and turn on its custom integrator option in the editor
• How to use audiobus for getting the volume level from the microphone
• When doing pooling of objects, you can remove objects from the tree and disable visibility/process when they are inactive. When using that object, put it back in the tree and enable visibility/process. I miss just setting something active/inactive from Unity.
• There's a plugin, CurveMesh3D for drawing curves.
• There's a plugin, Jiggle Bones, for making parts of the skeleton jiggle around with fake physics.
• Use BoneAttachment3D to get the transform of a bone from a skeleton. Accidentally turning on "override pose" on that bone attachment can really mess things up though. And even when you turn it off, the bone doesn't go to its default transform, it just stays in some arbitrary place.

There might be a good way to morph one sound into another. I just don't know it. But fading one sound in while fading another out works quite well.

var min_vol: float = 0.1
var max_vol: float = 0.6
# $Player.chest_to_world() returns a float from 0.0 to 1.0
func _process(_delta: float) -> void:
   $BreatheOut.volume_db = linear_to_db(min_vol+($Player.chest_to_world()*(max_vol-min_vol)))
   $BreatheIn.volume_db = linear_to_db(max_vol-($Player.chest_to_world()*(max_vol-min_vol)))

Oh I also learned that I'm nearly motion sickness immune and that not everyone is. 😁