You can still attach to the Vue prototype in Vue 3, the syntax is a bit different though.

From the RFC: https://github.com/vuejs/rfcs/blob/master/active-rfcs/0009-global-api-change.md

// Vue 2
Vue.prototype.customProperty = () => {}


// Vue 3
app.config.globalProperties.customProperty = () => {}

I messed around in CodePen with Vue Next (Vue 3) and prototypes and it still works: https://codepen.io/drehimself/pen/GRZRqGE

You'd also have to convert Vue.component to use the method on the instance too.

In that video, Jeffrey is adding methods directly to the Vue prototype so you don't really have to use the composition API. You can just call the method directly just like he's doing in the video.