The process is pretty simple. “difference”, “subtract” or “hole” (tinkercad. don’t ask me why they call it that.) all do the same thing. It’s a boolean operation that removes one shape from another shape.

you create the shape and subtract the middle bit (a square slightly smaller than the adapter part itself.) Then put the adapter centered in the gap, and use a second boolean operation (usually called “join”, tinker cad calls this “grouping”… don’t ask.)

Sometimes you’ll see a third option in there called ‘intersect’, which will cut both parts down to only where they overlap, FYI.

So the process is to import your files, remove the bits that need to be removed… then join the adapter to the case, and export as an STL. the reason it’s important that the block your using to remove is slightly smaller is so that the adapter joins in a solid manner.

if you’re willing to give tinker cad another go, this video, while basic covers everything you need to do to get it done.

ETA: I think you’re going to have difficulty printing the combined part. the case appears intended to print with the back against the bed. you might be able to print it on edge, but the adhesion is going to suck and the nozzle dragging is likely to pull it off. you could also add supports, but then… well… support removal.