Method of passing value between parent components of vue2 and vue3 child components

// Parent parent passes msg value like child component<template>
    <Children :datum="'I am the data of the parent component'"></Children>
</template>
​----------------------------------------------------------------------------------
// Child component receives data passed by parent componentexport default {
  // Write method 1: Receive with array  props:['datum'],
  // Write method 2: Use object to receive, which can limit the received data type, set default values, verify, etc.  props:{
      datum:{
          type:String,
          default:'This is the default data'
      }
  },
  mounted(){
      ()// I'm the parent component's data  },
}

<template>
    <Children @method="method"></Children>
</template>
<script>
  import Children from './Children';
  export default {
    components: {
      Children
    },
    methods: {
      method(data) { // The data here is the parameter passed by the child component. If the parameters have multiple parameters, you can use the ... syntax to get the parameters        (data);// Parameters passed by subcomponents      }
    }
  };
</script>
​----------------------------------------------------------------------------------
// Subcomponent Pass to parent component dataexport default {
   methods: {
      childMethod() { // The child component obtains the method passed by the parent component through $emit, and then carries the data to the parent component        this.$emit('method',"I am a child component");
      }
    }
}

// Parent component<template>
    <Children ref="child"></Children>
</template>
export default{
    import Children from './Children'
    export default{
        components:{
            Children 
        },
        mounted:{
            //Calling child component method, pay attention to distinguishing here that child is the name of ref           this.$(val)  // Find the child component through $refs and find the method to execute        }
    }
}

<template>
  <Children :title="I am the parent component"  ref="childrenRef" @method="methodChildren"></Children >
</template>
<script lang="ts">
import Children from "./"
import { defineComponent, ref } from "vue"
export default defineComponent({
  components: {
    Children ,
  },
  setup() {
    let msg = ref("")
    let childrenRef = ref() // Get instance of child component through ref    let fun = () =>{
    	()// How to use child components    }
    let methodChildren = (val) => {
       = val // Here val gets the value passed by the child component    }
    return {
      msg,
      methodChildren,
    }
  },
})
</script>

<template>
  <!-- Click to call the parent component method -->
  <button @click="fatherMethod">Click</button>
</template>
<script lang="ts">
import { defineComponent } from "vue"
export default defineComponent({
  name: "Children",
  props: {
    title: {
      type: String,
    },
  },
  setup(props, {emit}) {
    const fatherMethod= () => {
      emit("method", "Passing value to parent component")
    }
    const fatherFun= () => {
      ("I'm a child component method")
    }
    return {
      fatherMethod,
    }
  },
})
</script>