How to make an extruded side become a separate part blender?

Hello, I’m new to blender. So I don’t know how to use it very much. I’m trying to make a book shelf and I want the shelf part(which is extruded) do be a seperate part.

If someone can tell me how then I’d be happy!
Thank you

on edit mode select that faces click p and do Selection (Seperate Selection)

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THANK YOU!! I needed this! Thank you soo much!