UnityのEditorスクリプトでXR Plug-in ManagementのPlug-in Providersをセットする方法です。
Assemble Definitionを適切に設定
using UnityEngine;
using System;
using System.Linq;
using UnityEditor;
using UnityEditor.XR.Management;
using UnityEngine.XR.Management;

public class EnablePluginProviders : MonoBehaviour
{
    [MenuItem("Tools/Enable XRPlugin")]
    public static void EnableOpenXR()
    {
        EnableXRPlugin(BuildTargetGroup.Standalone, typeof(UnityEngine.XR.OpenXR.OpenXRLoader));
        EnableXRPlugin(BuildTargetGroup.Android, typeof(UnityEngine.XR.OpenXR.OpenXRLoader));
        EnableXRPlugin(BuildTargetGroup.VisionOS, typeof(UnityEngine.XR.VisionOS.VisionOSLoader));
    }

    public static void EnableXRPlugin(BuildTargetGroup buildTargetGroup, Type loaderType)
    {
        // Get the current XRGeneralSettings instance
        XRGeneralSettings xrGeneralSettings = XRGeneralSettingsPerBuildTarget.XRGeneralSettingsForBuildTarget(buildTargetGroup);

        if (xrGeneralSettings == null)
        {
            Debug.LogError("XRGeneralSettings is null. Make sure XR Plugin Management is installed and configured.");
            return;
        }

        // Get or create the XRManagerSettings instance
        XRManagerSettings xrManagerSettings = xrGeneralSettings.AssignedSettings;
        if (xrManagerSettings == null)
        {
            xrManagerSettings = ScriptableObject.CreateInstance<XRManagerSettings>();
            xrGeneralSettings.AssignedSettings = xrManagerSettings;
        }

        // Enable the specified loader if it's not already enabled
        if (!xrManagerSettings.activeLoaders.Any(loader => loader.GetType() == loaderType))
        {
            var xrLoader = ScriptableObject.CreateInstance(loaderType) as XRLoader;
            xrManagerSettings.TryAddLoader(xrLoader);
        }

        // Set the initialization mode to OnDemand or Automatic as needed
        xrManagerSettings.automaticLoading = true;
        xrManagerSettings.automaticRunning = true;

        // Save the changes
        EditorUtility.SetDirty(xrGeneralSettings);
        EditorUtility.SetDirty(xrManagerSettings);
        AssetDatabase.SaveAssets();

        Debug.Log($"{loaderType.Name} has been enabled successfully for {buildTargetGroup}.");
    }
}