How would I add data on a button press to the either the root or a component of Vue 3?

My main problem is that I’m trying to reactively add data to either the Vue root or a Vue component. The reason this is an issue is because once my Vue app instance is mounted, (using app.mount()), I am unable to reactively add data to the Vue application. I am trying to use Vue along with vanilla JS as I am a novice with the framework. I guess what it really comes down to is… is there any event I could trigger or object I could call in vanilla JS to insert new data into lets say an array of objects within the Vue app instance?