how do i access another saved scene through a script?

A "Pause" menu basically means that you want to temporally pause/stop the game an show a screen. Doing this with another scene can lead to complications.

A better way would be to create a UI Canvas with all the elements that you need and then create a prefab from it. The UI Canvas/Prefab would be disabled by default.

You could then add the prefab to all scenes. To enable/disable the pause menu you could have a script with something like this

 public GameObject pauseMenu;
 
 void EnablePauseMenu(bool enablePauseMenu)
 {
     if (pauseMenu != null)
     {
         pauseMenu.SetActive(enablePauseMenu);
         // setting Time.timeScale pauses (0)/unpauses (1) the game
         Time.timeScale = (enablePauseMenu ? 0 : 1);
     }
 }