Depends how you define as "already built".
The design is mostly done if not finish. The only wiggle room are spec and components. Which can change depending on whether it is single sourced and market demands. For example as per court Document during Apple Vs Qualcomm Case, Apple state they required Modem Supply Agreement to be signed and samples to be ready by no later than July for their next year's iPhone. ( That of course only applies to new Modem )
Even the Apple Silicon for next year's iPhone are already finished.
Yes, that means they are already working on iPhone 14.
And that is why the widely reported Rumours of Apple deciding not to ship the iPhone SE Plus in the last minute by certain famous twitter user or so called leaker are complete horse shit.