Nope, it does not. Only the trigger phrase (“Hey Siri“) is recognised locally on the device (according to Apple), everything that comes after that is sent as audio recording snippet to Apple servers where the actual speech recognition happens.
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.
> 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.
Sadly, offline dictation is a separate feature, it is not the speech-recognition used for Siri.
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. "
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...