Upgrading Process for my new iPhone

Hello, I have an iPhone XR and will be upgrading to the iPhone 15. I want to know what is the seamless way to upgrade my phone. Should I go to the apple store and buy the new phone? How will transferring all the data on my old phone to the new phone work? How will this affect my cellular plan with Verizon? Should I just go to Verizon to upgrade my phone? Thanks in advance.