Okay, first--in case you might not have noticed--I did qualify my opening gambit.
"Hi. My first Mac was the original 1984 Mac so I sort of know Macs." <--- emphasis on the 'sort of.' My husband encouraged me to buy that first Mac because he knew how un-technical my brain is and that I would never manage the IBM PC I had my eyes on!
So yes, you are correct--it is Sierra, not High Sierra, and I didn't even notice there was a difference.
Also, the file is .rmgc file I have is 1.3 megabytes so you are correct! It is empty.
I will check with my SIL and see what info she needs to create the file. All I know is that she was trying to send me only my own info, since her files have tens of thousands of people in them, and wasn't sure how to do it. I am pretty sure we're talking about a GEDCOM file.