Inside each project, you can see "Change This" compositions . Insert your design inside these compositions and screens will be updated in the overall composition.
All compositions are more than 4k format, this is done so that you can position the devices in the scene and crop as you need.
To move or transform layers, first convert them to a separate composition, select the layers and run «Layer - Pre-compose...» from top menu
Then you can crop the scene, select «Composition - Composition Settings» from top menu and set the size (recommended sizes for 4K 4096 x 2160 or 3840 × 2160).
Use «Composition - Add to Media Encoder» to export it to any format.