How do I transfer the information from my old phone to my new one if I can’t turn the old one on?iphone 15

If the phone isn't functional, the only way to move your data to the new phone would be if you were routinely backing your old phone up to iCloud. If you didn't back up your phone to iCloud and your old phone is broken, you can't move the data to your new iPhone.

Did you use iCloud backup? If you did, see this link about setting up a new iPhone from iCloud backup --> Use iCloud to transfer data from your previous iOS or iPadOS device to your new iPhone, iPad, or iPod touch - Apple Support