UX Process: How to not make this waterfall [ double diamond ]

In short, doesn't the double diamond or any other design process consider to be a waterfall? I mean what the hell do developers do when designers conceptualize and plan out the product?

I have a few follow up questions depending on the answer. Also don't want to blast this forum with a billion questions.

But what should dev do while ux designers go through the (beginning of the) design process