Working with UX/UI as a Software Developer

Hi everyone, I'm looking forward to collaborating on my first mobile and web app with a designer (UI/UX). I'm more of a back end guy (20+ years programming), and have minimal front end and mobile experience.

What is the usual or preferred method of transferring the work produced by a designer to a web or mobile app? Do I have to recreate their designs programmatically for each platform? Or is CSSs, HTML and images assets produced by the designers helpful to alleviate the integration work? Help!

I'm thinking of using React Native or Flutter for a fairly simply application (not a game or anything CPU/GPU intensive).

Any feedback would be greatly appreciated.

Thanks!