PBXProject.WriteToFile

Записывает содержимое проекта в указанный файл.

using UnityEditor;
using System.IO;
using UnityEditor.Callbacks;
using UnityEditor.iOS.Xcode;

public class Sample_WriteToFile  
{
    [PostProcessBuild]
    public static void OnPostprocessBuild(BuildTarget buildTarget, string pathToBuiltProject)
    {

        // Stop processing if build target is not iOS
        if (buildTarget != BuildTarget.iOS)
            return;

        // Initialize PBXProject by specifying the project path
        string projectPath = PBXProject.GetPBXProjectPath(pathToBuiltProject);
        PBXProject pbxProject = new PBXProject();

        // Read project information from the project file
        pbxProject.ReadFromFile(projectPath);

        // Get the target GUID you want to make modifications on
        string mainTargetGuid = pbxProject.GetUnityMainTargetGuid();

        // Modify the project, such as adding a build propety
        pbxProject.AddBuildProperty(mainTargetGuid, "exampleProperty", "exampleValue");
        
        // Apply changes to the pbxproj file
        pbxProject.WriteToFile(projectPath);
    }
}