iOS 64-bit support is implemented with the scripting backend called IL2CPP. It runs with your C# code.
The IL2CPP runtime combines an ahead of time compiler with a virtual machine to convert assemblies to C++ while leveraging standard platform C++ compilers to produce native binaries. The net result is significantly higher performance, platform compatibility and maintainability.
IL2CPP is the only scripting backend to support deploying to ARM 64-bit on iOS, and is thus mandatory to deploy to the Apple App Sstore for releasing new apps.
Select IL2CPP in the Scripting Backend drop-down menu on the Player window.
By default it will build for Universal architecture (including both ARM64 and ARMv7); if needed you can switch to specific architecture on the Player window. There are a number of things that should be done before your application is up and running in 64-bits:
Method not found: 'Default constructor not found...ctor() of System.ComponentModel.Int64Converter'.