SceneLoader.cs

using UnityEngine;
using UnityEngine.SceneManagement;

public static class SceneLoader {
    public static float SceneLoadProgress => loadStatus == null ? 0f : loadStatus.progress;
    static AsyncOperation loadStatus = null;
    const int mainMenuIndex = 0;
    const int gameIndex = 2;
    const int loadingIndex = 1;

    public static void LoadMainMenu() => StartLoadScene(mainMenuIndex);
    public static void LoadGame() => StartLoadScene(gameIndex);
    public static void ReloadScene() => StartLoadScene(SceneManager.GetActiveScene().buildIndex);

    async static void StartLoadScene(int buildIndex) {
        // Load the loading scene
        SceneManager.LoadScene(loadingIndex);
        // Force a wait so that the loading scene shows
        await Awaitable.WaitForSecondsAsync(0.1f);
        // Load target scene and save progress for loading scene
        loadStatus = SceneManager.LoadSceneAsync(buildIndex);
        loadStatus.completed += OnCompleteLoad;
    }
    static void OnCompleteLoad(AsyncOperation _) {
        loadStatus.completed -= OnCompleteLoad;
        loadStatus = null;
    }
}

public class LoadSceneStatus {
    public float Progress {get; private set;}
    public void SetProgress(float value, object setter) {
        if (setter is SceneLoader) {
            Progress = value;
        }
    }
}