Connect your iPhone to your iMac with a USB or USB-C cable
Unlock your iPhone and tap Trust This Computer if prompted
Open Finder on your iMac
Select your iPhone from the Finder sidebar
Click Trust if prompted on the iMac
Choose the content you want to sync
Click Apply or Sync
Wait for the sync to finish
Eject your iPhone safely before disconnecting
Use Wi-Fi syncing by enabling Show this iPhone when on Wi-Fi in Finder
Keep both devices on the same Wi-Fi network for wireless syncing