thank you for the feedback! Is it possible to extract enemies from other files not given in the initial zip file? And I plan to change the shape of the menu is that a significant modification?
The enemies provided in the assets are made to work with a variety of scripts in the assets provided, so in that regard it is not possible to extract other fully formed enemy assets that will work in the project. It is however possible to drag sprites into unity projects to use in creating new enemies with new visuals. You can craft a new enemy by duplicating one of the existing enemy prefabs and then editing it to make it behave differently and swapping sprites to make it look different.
The shape of the menu alone would not be significant. What would be significant is modifying the appearance of the entire menu, such as swapping out new sprites for everything and providing a new background for the menu. See the Nester game from the examples to see how they created an entirely new main menu background by hand: Optional: 2D Shoot Modification Examples - FS23-CAS-117-730 - Games and Interactivity (msu.edu) as an example, though you could also pull together a set of various images to make up a new interesting main menu background.