How do I ensure complete data transfer to my new iPhone?

Hello finman1957,

If you still have your Old iPhone try the Quick Start Method after seeing this Apple Support page.

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

OR

If you don't have your old iPhone your can use your iCloud account to make the transfer.

Use iCloud to transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

“How do I ensure complete data transfer to my new iPhone?: I installed my SIM card in my new phone. My pictures have transferred but not all my contacts and Apps .”

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Transferring to a New iPhone?:

If you a transferring to a new iPhone, then this is the link to use: Use Quick Start to Transfer Data to a new iPhone, iPad, or iPod touch - Apple Support. Use Quick Start to transfer data. If you have iOS16, then the iOS cannot be downgraded (and as a result, things would no be just like your iPhone 8 Plus).