TurboCocoa Software has released a native UI tool for Firemonkey in Delphi 10 Seattle which allows you to design and use the native layouts for Android, IOS, and OSX. Firemonkey itself provides an OpenGL based UI (with some native controls). There have also been third party options to use native controls via DPF IOS, DPF Android, TMS iCL, and TMS mCL. TurboCocoa is a new tool that enables a different approach (and one that some competing cross platform tools use).TurboCocoa allows you to create Storyboards in XCode for IOS and OSX and then write your code in Object Pascal. Or you can create your Layouts in Android Studio and write your code in Object Pascal. If you love object pascal but you want to have a 100% native platform UI you can achieve that with this solution. The draw back is that you have to build your UI separately for each platform. It should be pretty easy to re-use object pascal logic code across platforms however. Consider TurboCocoa just another tool in your Object Pascal toolchest. It enables windowless agent apps and document apps on OSX, multiple activity apps on Android, and split view apps on IOS. TurboCocoa is currently in beta but you can sign up and try it out.
FMXExpress.com has over 700 tips and tricks for building cross platform apps on Android, IOS, OSX, Windows, and Linux with a single codebase and single UI using the FireMonkey framework found in Delphi and C++Builder.
FMXExpress covers top cross platform secrets for FireMonkey in C++Builder and Delphi XE5, XE6, XE7, XE8, Seattle, Berlin, Tokyo, Rio, and Sydney.