class SceneManager: MonoBehaviour
{
  // Attach prefab via Unity Editor.
  [SerializeField] private GameObject playerPrefab;

  [SerializeField] private Vector3 spawnPoint;

  [SerializeField] private SomeClass otherStuff;

  Awake()
  {
    var playerInstance = Instantiate(playerPrefab, spawnPoint, Quaternion.identity);

    // Send reference to player where it belongs:
    otherStuff.Player = playerInstance;
  }
}