Add Buy Now Button in WooCommerce Without Plugin

Add below code in functions.php file.

Buy and empty Cart 

// Add Buy Now button on product page
add_action( 'woocommerce_after_add_to_cart_button', 'add_buy_now_button' );

function add_buy_now_button() {
    global $product;
    echo '<a href="' . esc_url( wc_get_checkout_url() ) . '?buy-now=' . $product->get_id() . '" class="button alt">Buy Now</a>';
}

// Handle Buy Now button redirect to checkout page
add_action( 'template_redirect', 'handle_buy_now_redirect' );

function handle_buy_now_redirect() {
    if ( isset( $_GET['buy-now'] ) ) {
        // Remove all products from the cart
        WC()->cart->empty_cart();
        
        // Add the specified product to the cart
        $product_id = intval( $_GET['buy-now'] );
        WC()->cart->add_to_cart( $product_id );
        
        // Redirect to checkout page
        wp_safe_redirect( wc_get_checkout_url() );
        exit;
    }
}

Buy Product + All product on Cart 

// Add Buy Now button on product page
add_action( 'woocommerce_after_add_to_cart_button', 'add_buy_now_button' );

function add_buy_now_button() {
    global $product;
    echo '<a href="' . esc_url( wc_get_cart_url() ) . '?buy-now=' . $product->get_id() . '" class="button alt buy-now-button">Buy Now</a>';
}

// Add CSS to change the color of the button
add_action( 'wp_enqueue_scripts', 'custom_buy_now_button_styles' );

function custom_buy_now_button_styles() {
    echo '<style>
        .buy-now-button {
            background-color: blue !important;
            color: white !important;
        }
    </style>';
}

// Handle Buy Now button redirect to cart page
add_action( 'template_redirect', 'handle_buy_now_redirect' );

function handle_buy_now_redirect() {
    if ( isset( $_GET['buy-now'] ) ) {
        // Add the specified product to the cart
        $product_id = intval( $_GET['buy-now'] );
        WC()->cart->add_to_cart( $product_id );
        
        // Redirect to cart page
        wp_safe_redirect( wc_get_cart_url() );
        exit;
    }
}

#Buy Product +Shop Page to Singal product page+ All product on Cart +

// Add Buy Now button on product page
add_action( 'woocommerce_after_add_to_cart_button', 'add_buy_now_button' );
add_action('woocommerce_after_shop_loop_item', 'add_buy_now_button');

function add_buy_now_button() {
    global $product;
    echo '<a href="' . esc_url( wc_get_cart_url() ) . '?buy-now=' . $product->get_id() . '" class="button alt buy-now-button">Buy Now</a>';
}



// Handle Buy Now button redirect to cart page
add_action( 'template_redirect', 'handle_buy_now_redirect' );

function handle_buy_now_redirect() {
    if ( isset( $_GET['buy-now'] ) ) {
        // Add the specified product to the cart
        $product_id = intval( $_GET['buy-now'] );
        WC()->cart->add_to_cart( $product_id );
        
        // Redirect to cart page
        wp_safe_redirect( wc_get_cart_url() );
        exit;
    }
}

Redirect to cart not checkout + Buy Product + All product on Cart 

// Add Buy Now button on product page
add_action( 'woocommerce_after_add_to_cart_button', 'add_buy_now_button' );

function add_buy_now_button() {
    global $product;
    echo '<a href="' . esc_url( wc_get_cart_url() ) . '?buy-now=' . $product->get_id() . '" class="button alt">Buy Now</a>';
}

// Handle Buy Now button redirect to cart page
add_action( 'template_redirect', 'handle_buy_now_redirect' );

function handle_buy_now_redirect() {
    if ( isset( $_GET['buy-now'] ) ) {
        // Add the specified product to the cart
        $product_id = intval( $_GET['buy-now'] );
        WC()->cart->add_to_cart( $product_id );
        
        // Redirect to cart page
        wp_safe_redirect( wc_get_cart_url() );
        exit;
    }
}

2. Add CSS Code.

#wpamit-adding-button {
      margin-left:20px;
	background-color:blue
}
 
@media only screen and (max-width: 600px) {
 #wpamit-adding-button {
      margin:10px;
}
}