Sorry for long time to reply. My PSU got burned so I had to go back to console peasant realm a bit.
When I began to write this project I had no idea how to write godot FPS myself. So If you read code - you will be confuzed since it is overengeneering mess. Main part - you just attach 3d camera to 3d rigid body - and all work is done. After that you just add features.
Take a look at this addon for weapons and reloading:
https://godotengine.org/asset-library/asset/58
Saving data between scenes is usually done with Singletons:
http://docs.godotengine.org/en/stable/learning/step_by_step/singletons_autoload....
The forum for godot exists https://godotdevelopers.org/forum/
Commenting with "how do I do [insert basic stuff]" are kinda not fun to read in general ). It looks like you make fun of game developers (I suppose you are one).
Weapon model is exported from blender, animated with AnimationPlayer, a child of player's rigid body (camera as well). Script on player's rigid body switches the weapon animation.
Crystal have a script that checks if player touches it, then it does +1HP +1AMMO and deletes itself.
