Best Way to Support Different Devices in SwiftUI?

Hi, I have pretty much finished my app's layout but realized I needed to scale it for different devices. I have read online that hardcoding values (esp in frames) is a big no-no, and GeometryReader should be heavily utilized. Also was recommended ViewThatFits. The problem is, I want the app to look the exact same across all devices. What is the best way to get started?

Also, when testing, do I only have to test on an iPad and iPhone or are the dimensions significantly different amongst each class of devices?