Sadly it’s not doing it in the background and instead opens the app with the information pre-filled. I almost exclusively enter transactions via a shortcut that enters transactions directly. While walking out of the supermarket all I say is "(Hey) Siri, new supermarket transaction" - "Amount?" - "23.56" - "… 23.56, Supermarket, ".
I thought my own shortcut was obsolete now (which I would’ve loved!). I had spent days building one that talks to the api directly and eventually made another one for file import. But as long as it’s not doing it in the background I’ll stick with these.