Yes.

TS won't allow you to use this type without defineComponent or any or harder things

export let a = {
  data: () => ({ a: 1 }),
  methods: {
    x() {
      this.a
      // ^! Property 'a' does not exist on type '{ x(): void; }'.(2339)
    },
    y(this: { a: number }) {
      this.a // this works
    }
  }
}

In JS almost no, defineComponent is a no-op (e => e)

The quick explanation: defineComponent() predates

If you need to create multiple components I would highly recommend using Single File Components (SFC)

Here you define a new component as (inside the <script> tag):

import { defineComponent } from 'vue'
export default defineComponent({
  // ...
})

(or export default {} if not using TypeScript)

For the main app component you would do this:

import { createApp } from "vue";
const app = createApp(App)
app.mount('#app')

OR just like this, if you don't need to extent Vue with vue-router, Vuex etc.

import { createApp } from "vue";
createApp(App).mount('#app')