Howdy Barney,
You ask an excellent question about whether syncing your iPhone with your Mac will files be deleted from either of the devices. Let me try to give you the best explanation that I can.
This article gives good basic information about syncing an iOS device such as your iPhone with a computer such as your Mac -
Sync your iPhone, iPad, and iPod with iTunes using USB - Apple Support
You will note that most information goes one way - from the computer to the iPhone. Example - if you have ripped a CD into iTunes on your Mac, it would remain on the computer after syncing and could be put on the iPhone if desired without loss to the computer.
Photos work in a slightly different way, since it is possible to import photos onto your computer from the iPhone as well as put photos in your picture collection on your Mac onto the iPhone. This article shows how to import photos to the computer -
Import photos and videos from your iPhone, iPad, or iPod touch to your Mac or Windows PC - Apple Support
This article describes the process for going the other direction -
Sync photos to your iPhone, iPad, and iPod touch in iTunes - Apple Support
I hope this has given you useful information about syncing your iPhone.
Thanks for using Apple Support Communities.
Best,
Brett L