How to use VUE components in a regular JS file that adds elements to the page in #app?

Question

Vue.js

I have a project on VUE 3 and I made the modal window a separate JS file (not as a vue component ).
How can I use my VUE component in a simple JS file that adds a modal window to the page and does it in the VUE root element - #app ?

In react, I would just use ReactDOM.render - I need some kind of analogue here