https://www.cnet.com/news/the-new-ipad-pro-can-now-charge-your-iphone/

The way it works is the same as charging your phone with a Mac, you just use a connector cable to plug the smartphone into the tablet's USB-C port.

While Apple added the USB-C port to its new iPad Pros, it eliminated both the Lightning connector and the 3.5mm headphone jack. The iPad's lack of an audio output is part of the explosion in Bluetooth headphones. Eliminating those two ports means the new device is now 15 percent slimmer than the previous version.