With a typical design-led approach, you define the UI specifications before development begins. And to offer a consistent UX, designs are created using a common design system or language in a tool like Adobe XD or Sketch.
Developer hand-off is the point when design specifications are ready for development. Since designers and developers use different tools, it remains a challenge. There are bridge tools to make hand-off easier but are tedious. Developers use these tools to extract the visual specifications and replicate these in code. Visual specifications may include app layouts, size, spacing, theme colors, and illustrations (if present). App Builder can completely replace this workflow and avoid coding it from scratch. And ensure that the resulting app is pixel perfect.