The build number of the bundle
It is saved to the 'CFBundleVersion' field in the built app's info.plist file.
                        
using UnityEngine;
using UnityEditor;
public class BuildNumberExample
{
    // This example will create a Menu Item called "Increase Minor Build Number" under "Examples" submenu which when pressed will get the current "Build" property and increase the Minor version by 1 
    // Assuming the "Build" property follows the format Major.Minor.Patch (for example 2.0.5 will be increased to 2.1.0),
    [MenuItem("Examples/Increase Minor Build Number")]
    public static void IncreaseMinorBuildNumber()
    {
        // Get the current build number value
        string currentBuildNumberString = PlayerSettings.iOS.buildNumber;
        // Separate the build number string to Major, Minor and Patch numbers
        string[] separatedBuildNumbers = currentBuildNumberString.Split('.');
        // Parse the Major and Minor version numbers to integer values
        int majorVersionNumber = int.Parse(separatedBuildNumbers[0]);
        int minorVersionNumber = int.Parse(separatedBuildNumbers[1]);
        // Increment the Minor version number
        minorVersionNumber++;
        
        // Update the buildNumber property with the new values:
        // - keep the old Major version number;
        // - update Minor version number;
        // - replace Patch version number with 0, since Minor version was updated. 
        PlayerSettings.iOS.buildNumber = string.Format("{0}.{1}.{2}", majorVersionNumber, minorVersionNumber, 0);
    }
}