Proceedure buying a new iPhone SE(3)

It's been a while since I bought a new iPhone and the buying and activating process seems to have changed. I thought if you were to buy online, you would go thru the color, capacity, etc, and select your carrier. Once the carrier was selected you would then input your phone number, PIN and your phone would be sent ready to use.

Now in the online process, there seems to be no set up for your carrier account, other than selecting a carrier or no carrier.

So how is it done now. Would I choose the color, capacity, my carrier, which is At&T, the other options, then simply checkout and set up the carrier settings when the phone arrives??

I already have a working iPhone SE 1st generation with a number and AT&T account. Do I simply have the the new phone close to the old phone and transfer carrier setting that way.

Any help appreciated..