I don't have a USB-C cable or USB-C adapter.
How do I sync my iPhone 5s with my 2015 MacBook? I thought I could do it over WiFi?
2015 MacBook's don't have USB ports
I also have a MacBook Pro but I don't want my iTunes library on this machine.
To Sync your iPhone via WiFi your iPhone needs to be connected with a cable to the MacBook once. Then in iTunes there will be an option on the main screen for the iPhone where you can select "WiFi Synchronization".
This will make it possible for the iPhone to be synced when on the same network and manually when navigating to Settings > General > Wi-Fi iTunes Synchronization.
Download VLC app on your iPhone.
In that it'll provide an option to sync via same wifi network. It'll give you a http address you can enter on your MacBook, then simply drag and drop any music to the website and it'll send them over the same wifi network to your iPhone.
Relatively quick and very easy to do.
You need to connect the iDevice to iTunes with the Dock/Lightning to USB cable (you can use an adapter or Thunderbolt device that has USB ports) and enable WiFi Sync in the Summary section of the iDevice. There is no way to enable this setting wirelessly. Then it will auto-sync when the iDevice is plugged in and connected to WiFi. Syncs can also be started manually over WiFi.