Using react-router-dom in my project,how to make menu is above on main

I'm a beginner of react, I using create-react-app created a react project.When i add react-router-dom in project, add <BrowserRouter> in index.js, render my menu in Header.js, but the page rendered is not expected as I found menu is on the bottom of page, and when i click menus, page body is above menus. My code is here, Can u help me?

//index.js
import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';

import {BrowserRouter} from 'react-router-dom';

ReactDOM.render(
    <BrowserRouter >
        <App />
    </BrowserRouter >,
  document.getElementById('root')
);

//App.js
import styles from "./App.module.scss"
import ErrorBoundary from "./components/error-boundary/ErrorBoundary";
import Header from "./components/common/Header";
import {Route, Routes} from "react-router-dom";
import {useEffect, useState} from "react";

function About() {
    return (
        <div>about me.</div>
    );
}

function Clock() {
    const [tick, setTick] = useState('');
    useEffect(() => {
        let timerID = setInterval(() => {
            let now = new Date();
            setTick(now.toLocaleTimeString());
        }, 1000);
        return () => clearInterval(timerID);
    }, [tick])

    return (
        <div>{tick}</div>
    );
}

function Home() {
    return (
        <div>my home.</div>
    );
}


function App() {
    const navs = [
        {id: 1, name: 'Home', path: '/', element: <Home/>},
        {id: 2, name: 'About', path: '/about', element: <About/>},
        {id: 3, name: 'Clock', path: '/clock', element: <Clock/>},
    ];
    return (
        <>
            <Routes>
                {
                    navs.map((nav) => {
                        return (
                            <Route key={nav.id} path={nav.path} element={nav.element}/>
                        );
                    })
                }
            </Routes>

            <div className={styles.container}>
                <ErrorBoundary>
                    <Header navs={navs}/>
                </ErrorBoundary>
            </div>
        </>

    );
}

export default App;

//Header.js
import {Link} from "react-router-dom";
import styles from './Header.module.scss';

import React, {Component} from 'react';

class Header extends Component {
    render() {
        return (
            <>
                <ul className={styles.menus}>
                    {
                        this.props.navs.map((nav) => {
                            return (
                                <li key={nav.id} className={styles.menu}>
                                    <Link to={nav.path}>{nav.name}</Link>
                                </li>
                            );
                        })
                    }
                </ul>
            </>
        );
    }
}

export default Header;

Just put the Header JSX above the Routes. The Route component renders the component when its path matches the current URL.

The UI hierarchy should be:

function App() {
    const navs = [
        {id: 1, name: 'Home', path: '/', element: <Home/>},
        {id: 2, name: 'About', path: '/about', element: <About/>},
        {id: 3, name: 'Clock', path: '/clock', element: <Clock/>},
    ];
    return (
        <>
            <div className={styles.container}>
                <ErrorBoundary>
                    <Header navs={navs}/>
                </ErrorBoundary>
            </div>

            <Routes>
                {
                    navs.map((nav) => {
                        return (
                            <Route key={nav.id} path={nav.path} element={nav.element}/>
                        );
                    })
                }
            </Routes>
        </>

    );
}