Siri is a web app. It requires an internet connection to operate, because everything you say to it is sent to Apple's servers to be processed.

And, while I find "Talk to text" on my iPhone to be almost flawless, Siri is severely limited by which apps support it.

For example, I can't tell Siri to "pause the music" or "skip this song" because I use Spotify, and Spotify and Apple are fighting, so Siri pretends not to know Spotify is playing music.

So Siri generally works better when controlling Apple apps, because the integration is built in. Maybe that's part of the issue you are having?