How does one aquire such knowledge?

You see how in Phantom Forces where theres no backpack or toolbar and you have weapons? How do you do that?

They have disabled the ROBLOX Backpack by using this line of code:

game.StarterGui:SetCoreGuiEnabled("Backpack", false)

Then they made their own backpack.

Hope this helps

They wrote the code

game.StarterGui:SetCoreGuiEnabled("Backpack", false)

Then made their own through a GUI. (Improving off of Unlimitus)