Consolidando o conhecimento: 01. Flexbox | CSS: dispondo elementos com Flexbox e Grid

Olá, seguem os arquivos até o momento: index.html:

<!DOCTYPE html>
<html lang="pt-br">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>HZC | Home</title>
    <link rel="preconnect" href="https://fonts.googleapis.com">
    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
    <link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Open+Sans:wght@400;600;700&display=swap">
    <link rel="stylesheet" href="assets/css/reset.css">
    <link rel="stylesheet" href="assets/css/estilos.css">
</head>
<body>
    <header class="cabecalho">
        <button class="cabecalho__menu">Menu</button>
        <img class="cabecalho__logo" src="assets/img/logo.svg" alt="Logotipo da HZC">
        <button class="cabecalho__notificacao">Notificação</button>
    </header>

    <main>

    </main>

</body>
</html>

estilos.css:

body {
    background-color: #1D232A;
    font-family: 'Open Sans', sans-serif;
    color: #ffffff;
}

.cabecalho {
    display: flex;
    justify-content: space-between;
    align-items: center;
    background-color: #15191C;
    padding: 8px 16px;
    box-shadow: 0px 4px 4px rgba(0, 0, 0, 0.16);
}