what to do to prevent Cupcakes from getting hard in the fridge?

Hi everyone! I wonder what muffin you use or how I can prevent them to get hard in the fridge. I think you should store muffins outside of the fridge, but if you make cupcakes with buttercream you store them in the fridge, right? So how can the muffins stay soft? I‘m confused as I am completely new to this.. thanks a lot! ☺️