Mobile design guidelines and handoff practices? [on hold]

Backstory: I have no previous digital design experience but recently started working as a UI/UX designer for a small company. This is my first job.

Problem: We are currently working on a react native app and I'm unsure as to how to proceed with the design. I want the design to be consistent across all devices. My points of confusion are as follows:

  1. What screen dimensions do I design for? Do I need to redo my designs for different screens (say, for Pixel 2 and Nexus 4)? as they have different dimensions? or is there a way to have a single design dimension that works for (mostly) all devices?
  2. The developer I'm working with too doesn't know much about responsive design, so, while handing over the specs to the developer, what units should the layout dimensions be in? When should I use px and when should I use % values? I would want it to adapt to the screen size so that the developer too doesn't have to rewrite codes for different screens.