Answers
What happens if I make changes on my iPhone and on my Mac?
Changes can be made to the same family file on the iPhone and the Mac simultaneously. In fact, you could place your family file on multiple iPhones or iPads, make changes on each device and on the Mac, and then send the changes back to the original family file on the Mac.
For example: You might add 10 new people on the iPhone and edit data for 5 people and then add 10 other people on the Mac and edit 2 more people. When you send the changes from the iPhone back to the Mac, all 20 new people and 7 edits will appear on the Mac. At this point you would want to send the updated "master" family file (on the Mac) back onto the iPhone.
Note: changes could be lost if the same data has been edited on two different systems. I.e., you should avoid making changes to the same fields for the same person on different systems.