How do I transfer data from my iPhone 12 to my iPhone 15?

See this article:

Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

I assume that the iPhone 15 is new, and has not yet been set up. There are various methods to transfer data, but the Quick Start method seems to be the easiest for most users.

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

rlhatton1 Said:

"Data transfer: How do I transfer data from my iPhone 12 to my iPhone 15?"

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Use Quick Start:

Quick Start is a service provided by Apple for transferring to a new iPhone. Leave about a 40% free space.

If you were synchronizing anything with iCloud on the old phone, connecting to the same iCloud account and services with the new phone will cause that stuff to be synchronized onto the new phone.