Apple's Support resources state the following (bold added for emphasis):
You can use your iPhone 15 and later to charge your AirPods, Apple Watch or another small device that supports USB Power Delivery at up to 4.5 watts.
- If you have another device with a USB-C connector, connect it to your iPhone with a USB-C cable, such as the one included with your iPhone.
- To charge a device that has a Lightning connector, connect it to your iPhone using an Apple USB-C to Lightning cable.
- Charge your Apple Watch by connecting an Apple Watch Magnetic Fast Charger to USB-C Cable directly to your iPhone. You can also connect a Magnetic Charging Cable to your iPhone with the Apple USB-C to USB Adapter.
The full text can be found here:
Charge and connect with the USB-C connector on your iPhone - Apple Support
As your iPad requires a power source rated at least 12W to reliably charge, your iPhone is unable to output sufficient power to charge your iPad.