Hi pk_PK,
iTunes has been replaced by the Music app and Finder is now used to sync music from the computer to the iPhone.
So, the key is to make sure your Music library is up to date, and that you have a backup of your Mac and your iPhone before agreeing to sync the library to the iPhone. Sometimes, creating playlists of the music you want on your iPhone and syncing just the playlists makes the process a little simpler.
In addition to the Time Machine backup, we would suggest copying your Music folder to a different external drive.
Use external storage devices with Mac
Whatever you choose to sync from your library will replace whatever is on the iPhone, not the other way around.
You don't have to do this if you are content with what you have on your iPhone. You can purchase and download new music directly from the iTunes Store to the iPhone if you wish. The only time you would need to sync via USB is if you imported CD's into your computer and wanted them on your iPhone.
Here are some resources to help you with this transition:
What happened to iTunes?
Use the Finder to sync your iPhone, iPad, or iPod with your computer
Take care.