Merging two objects (I'm a beginner)

Hello, thanks in advance for any help. I have a little project I am trying to do-- I would like to merge a "Mag quadsafe universal adapter" with a Galaxy Fold 3 case. I have both STLs and i began playing with blender yesterday, and it's possibly a little above my head.

What it seems like the steps are from my (peprhaps simplistic) point of view:

• trim the openings on mag quad adapter to fit the camera lens and side buttons when it is perfectly centered on the case.
• delete a square the exact size of the magsafe quadlock adapter that is perfectly centered on the back of the phone case
• put the current model from the mag quad stl in thus square

The only thing I've been able to accomplish thus far is centering the mag quad adapter on top of the case. I cannot, for the life of me, figure out how to trim in to fit the camera and side button openings....

Any help is greatly appreciated, even if it's just a starting point.