How do I transfer data from my new iPhone back to my old iPhone and activate my old iPhone?

Be certain your “old iPhone” has an iOS version no less than your “new iPhone”.

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

Be certain what was your “new iPhone” is thoroughly cleansed after this process.

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Glad that answered your question.

Enjoy your Apple gear.