Hello,
One Layout is the actual physical keyboard layout. The one keyboard layout is the Windows keyboard layout.
Let me explain with an example:
You bought a laptop from the US. The laptop keyboard layout is English US. It looks like this on the laptop keyboard.
If you add a UK keyboard, it will behave like this - note the symbols on the number keys.
If you have set up EN-UK keyboard, once you press SHIFT+2 it will type "double quotes" despite on the keyboard @ is mentioned.
I hope this information helps. If you have any more questions, don't hesitate to reach out. Wishing you good luck!