The voice commands to start stop Siri dictation

Mike ~ Apple's Shortcuts app has a "Dictate text" action which offers options for when to stop dictation:

...Shortcuts can be triggered by saying: "Hey Siri <Shortcut name>. Shortcuts can also be set up as icons on the Home screen. If that interests you, I can post some links to free, third-party shortcuts which use that "Dictate text" action in various ways, such as saving to Notes.

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"Hey Siri" to start Siri --> Use Siri on all your Apple devices - Apple Support