ParentComponent.vue

<template>
  <div>
    <p> {{ data }} </p>
    <custom-input 
      :data="data" 
      @update:data="data = $event"
    />
  </div>
</template>

<script>
import CustomInput from './components/CustomInput.vue'
import { ref } from 'vue'

export default {
  components: {
    CustomInput,
  },
  setup () {
    const data = ref('hello')
    return {
      data
    }
  }
}