Bootstrap CSS class .navbar-toggler-icon

    <nav class="navbar navbar-expand-xxl navbar-light bg-light">
        <a class="navbar-brand" href="#">LOGO</a>
        <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
        </button>

        <div class="collapse navbar-collapse" id="navbarSupportedContent">
            <ul class="navbar-nav mr-auto">
                <li class="nav-item active">
                    <a class="nav-link" href="#">Home</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="#">Link 1</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="#">Link 2</a>
                </li>
                <li class="nav-item">
                    <a class="nav-link" href="#">Link 3</a>
                </li>
            </ul>
            <form class="d-flex">
                <input class="form-control me-2" type="search" placeholder="Search" aria-label="Search">
                <button class="btn btn-outline-primary" type="submit">Search</button>
            </form>
        </div>
    </nav>