Only that Siri doesn't happen on the iPhone, and it's easy to prove: try using Siri with aeroplane mode on.
There are rumours [0] that Apple might enable local speech-to-text at some point, but as of now, that’s not how it works.
[0] https://appleinsider.com/articles/18/11/15/apple-considering...
> You can use dictation instead of your keyboard to enter text with many apps and features that use the iOS keyboard.
> On iPhone 6s or later, and iPad, you can use dictation without being connected to the Internet. Earlier models of iPhone and iPad require an Internet connection.
Quote directly from Apple:
"The recording of the user’s spoken words is sent to Apple’s voice recognition server. If the task involves dictation only, the recognized text is sent back to the device. "
Source:
Page 69 in https://www.apple.com/business/site/docs/iOS_Security_Guide....
Siri is the service. Speech to text, while it can be improved with network access, works fine without it on current iOS.