References between vue components

I want to reference a dom,
on a component, but print it out as undefined,. How to reference this

<template>
    <div ref="a"></div>
    <show :dom="$refs.a"/>
</template>

show.vue

    export default {
    name: "show",
    props: {
        dom: String,
        params: Object,
    },
    mounted() {
        console.log(".........",this.dom); //undefinde
    },
};
Mar.20,2021

Sorry! I'm confused about the concept of ref.
to copy an official document:

important note about ref registration time: because ref itself is created as a rendering result, you can't access them during initial rendering-they don't exist yet! $refs is not responsive either, so you should not try to use it for data binding in templates.

if you want to access the dom structure of the parent component in the child component, you can pass it in by binding data.

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