How to use Vue 3 & Add Plugin Boostrap-vue?

For vue 3 you can use bootstrap-vue-3 Install: npm i bootstrap-vue-3 Config:

import { createApp } from "vue";
import App from "./App.vue";
import BootstrapVue3 from "bootstrap-vue-3";

import "bootstrap/dist/css/bootstrap.css";
import "bootstrap-vue-3/dist/bootstrap-vue-3.css";

const app = createApp(App);
app.use(BootstrapVue3);
app.mount("#app");

Bootstrap Vue is not yet ready for Vue 3.

To answer part of your question, Vue 3 changes the method for instantiating the application instance, including how plugins are registered.

For example...

import { createApp } from 'vue';
import Router from './router/Router';

createApp({/* options */}})
  .use(Router)
  .mount('#app');

You can read more about this at the official docs.

https://v3.vuejs.org/guide/instance.html

https://v3-migration.vuejs.org