Once data transfer is completed is my old iPhone unusable or will it still work the same as it did before the transfer?

Hi JS5121,

Thanks for reaching out to Apple Support Communities. It sounds like you have a question about transferring data to a new iPhone. We're happy to provide some information to assist.

You can use Quick Start to transfer new data: Use Quick Start to transfer data to a new iPhone, iPad, or iPod touch

You can also view all methods here: Transfer data from your previous iOS or iPadOS device to your new iPhone, iPad, or iPod touch

When transferring SIM cards, the cellular service on the previous device will no longer be active. This means no SMS text messages, calls or browsing without a Wi-Fi connection. If needed, the article here can help you swap sim cards: Remove or switch the SIM card in your iPhone

Cheers!