my 4th challenge, use flexbox entirely

I write the code mobile first. I have a hard time figuring how to efficiently change the display to the desktop design.

So i just add :

• Div to my header for left-side -Div to my main for the right-side and remove the footer section entirely (because somehow the footer section doesn't included in the div on the developer tools.

So i have a question and hoping for an answer so i can approach the same concept in more efficient way.

How can i change from mobile design to the desktop design more efficiently?