Hi Jorge,
The Plugin is a wrapper around cordova component https://github.com/kelter-antunes/cordova-plugin-speechrecognition, which uses the google speech API for android ad Apple's servers for iOS.
For that the internet is required.
Privacy information Google services: https://cloud.google.com/speech-to-text/docs/data-logging
Privacy information Apple: https://www.apple.com/privacy/features/
Regards,
Daniel