How to understand Vue's simple state management store mode

//store.js
export const store = {
    state: {
        todos:
            {text: 'Write Chinese homework', done: false},
            {text: 'Do math test', done: false}
        ]
    },
    addTodo(str){
        const obj = {text: str, done: false}
        this.state.todos.push(obj)
    },
    setDone(index){
        this.state.todos[index].done = true
    }
}

//A.vue
<template>
    <div class="A">
        I am component A<ul>
           <li v-for="(todo,index) in todos" 
           :key="index" :class="todo.done?'done':''" @click="setDone(index)">
           {{todo.text}}
           </li>
       </ul>
    </div>
</template>

<script>
import {store} from '../store/store.js'
export default {
    name: 'A',
    data(){
        return store.state
    },
    methods: {
        setDone(index){
            store.setDone(index)
        }
    }
}
</script>

<style scoped>
.A{
    background: red;
    color: white;
    padding: 20px;
}
.A li.done{
    background: green;
}
</style>

//B.vue
<template>
    <div class="B">
        <div>
            I am component B, enter the task in the input box below and add the task in component A</div>
        <input type="text" v-model="text">
        <button @click="addTodo">add todo</button>
    </div>
</template>

<script>
import {store} from '../store/store.js'
export default {
    name: 'B',
    data(){
        return {
            text: ''
        }
    },
    methods:{
        addTodo(){
            if(this.text){
                store.addTodo(this.text)
            }
        }
    }
}
</script>

<style scoped>
.B{
    background: yellow;
    padding: 20px;
}
</style>

//App.vue
<template>
  <div id="app">
    <A />
    <B />
  </div>
</template>

<script>

import A from './components/A.vue'
import B from './components/B.vue'

export default {
  name: 'App',
  components:
    A,
    B
  }
}
</script>