How to Add Filter or Hook for "Woocommerce_Add_To_Cart"

Hooks around add to cart for Woocommerce

Here below are the hooks involved in WC_Cart add_to_cart() method:

A) Before an item is added to cart:

Validation filter hook woocommerce_add_to_cart_validation

Item Quantity change filter hook woocommerce_add_to_cart_quantity (not with ajax)

Item Data change filter hook woocommerce_add_cart_item_data (not with ajax)

and some others related to "sold individually" products (see here)

A) After an item is added to cart:

Change cart Item filter hook woocommerce_add_cart_item

Add an event, action hook woocommerce_add_to_cart

To be clear in your case:

As you can see now woocommerce_add_to_cart is not a function but only an action hook.
Hook location: It's located inside WC_Cart add_to_cart() method (at the end of the source code).
When the hook is fired: It's fired once the WC_Cart add_to_cart() method is executed.
What is the purpose: To execute some custom code when this method is executed (event).

Regarding your code:

It should be better to use the dedicated filter hook woocommerce_add_to_cart_validation that will stop customer that want to add a new item to the cart if there is already a product in cart from a different vendor, displaying optionally a custom message:

add_filter( 'woocommerce_add_to_cart_validation', 'filter_add_to_cart_validation', 10, 3 );
function filter_add_to_cart_validation( $passed, $product_id, $quantity ) { 
    if ( WC()->cart->is_empty() ) return $passed;

    // Get the VendorId of the product being added to the cart.
    $current_vendor = get_wcmp_product_vendors($product_id);

    foreach( WC()->cart->get_cart() as $cart_item ) {
        // Get the vendor Id of the item
        $cart_vendor = get_wcmp_product_vendors($cart_item['product_id']);

        // If two products do not have the same Vendor
        if( $current_vendor->id != $cart_vendor->id ) {
            // We set 'passed' argument to false
            $passed = false ;

            // Displaying a custom message
            $message = __( "This is your custom message", "woocommerce" );
            wc_add_notice( $message, 'error' );
            // We stop the loop
            break; 
        }
    }
    return $passed;
}

Code goes in function.php file of your active child theme (or active theme) or in any plugin file.

Tested and works.

WooCommerce Add to cart, add second product from a custom select field

The problem was that I used global variable $product in foreach loop. So it was breaking loop logics and using global $product instead of desired array value. So all it takes to make the code work is changing foreach loop code to something like:

foreach ($products as $single_product) {
        $product_id = $single_product->get_id();
        $options[$product_id] = $single_product->get_name();

Otherwise the code is working fine, although I have to make a few remarks:

the code will only work if inserted via hooks which are called inside <form> ... </form> submitted by pressing add to cart button, i.e. woocommerce_before_add_to_cart_button, woocommerce_before_add_to_cart_quantity and woocommerce_after_add_to_cart_quantity (you can check it yourself in the template - https://github.com/woocommerce/woocommerce/blob/4.1.0/templates/single-product/add-to-cart/simple.php) for simple product and also a few hooks called inside variation loop vor variable product which can be found here - https://github.com/woocommerce/woocommerce/blob/4.1.0/templates/single-product/add-to-cart/variable.php
when inserted via woocommerce_before_add_to_cart_button input field is placed inbetween product stock text and add to cart button which is not the best place and it looks like you either have to edit template files or use JS to achieve better placement with current WC (4.1) hooks.
wc_get_products () function is rather slow. In my test it was 10 times slower than similar query made through $wpdb->get_results
- it is better to insert product_option_add_to_cart() custom function code into if (isset($_POST['chain_type'])) { ... } to prevent PHP notices in case there's no $_POST value for some reason

Woocommerce 3 Filter Single Variation Add to cart button html

Please try the following code. I haven't tested it but it supposed to work. Please try this on your dev environment first.

add_action( 'woocommerce_before_template_part', function( $template_name ) {
    if ( 'single-product/add-to-cart/variation-add-to-cart-button.php' === $template_name ) {
        ob_start();
    }
} );

add_action( 'woocommerce_after_template_part', function( $template_name ) {
    if ( 'single-product/add-to-cart/variation-add-to-cart-button.php' === $template_name ) {
        $content = ob_get_clean();
        $content = str_replace( '<div class="woocommerce-variation-add-to-cart variations_button">', '', $content );
        $content = str_replace( '</div>', '', $content );
        echo $content;
    }
} );

Adding content after add to cart button on woocommerce single page

Here, you need to echo content as it is add_action hook.

add_action( 'woocommerce_after_add_to_cart_button', 'add_content_after_addtocart_button_func' );
/*
 * Content below "Add to cart" Button.
 */
function add_content_after_addtocart_button_func() {

        // Echo content.
        echo '<div class="second_content">Other content here!</div>';

}

Woocommerce added to cart hook (after the product was successfully added to cart)

Answering this old question for Google Searchers:

The action woocommerce_add_to_cart is fired after an item is added to the cart.

and can be used like:

add_action( 'woocommerce_add_to_cart', function ()
{
  // your code here
});

related action woocommerce_cart_item_removed fires after an item is removed

Execute a piece of code before woocommerce_add_to_cart

Is not possible to make a redirection before add to cart in a simple way, but you can use woocommerce_add_to_cart_validation hook to avoid add to cart and display a custom notice with a linked button to your specific verification page.

Check that the correct meta_key for your verified status in the code is 'woo_VerifyStatus'

The code:

add_action( 'woocommerce_add_to_cart_validation', 'custome_add_to_cart_validation', 10, 1 );
function custome_add_to_cart_validation( $passed ){
    // When user is logged in in we get his verified status
    if( is_user_logged_in() )
        // Get user 'woo_VerifyStatus' postmeta value
        $verified_user = get_user_meta( get_current_user_id(), 'woo_VerifyStatus', true );

    // When user is not logged in we avoid add to cart and display a custom message
    if( ! is_user_logged_in() ){
        $message = __( "Please, you need to be registered and a verified user.", "woocommerce" );
        $button_text = __("Login or register", "woocommerce");
        $url = home_url('/gld/my-account');
        $message .= ' <a href="'.$url.'" class="login-register button" style="float:right;">'.$button_text.'</a>';
        $passed = false; // Set to false

        $message .= ' <a href="#" class="login-register button" style="float:right;">'.$button_text.'</a>';
    }

    // When is not a verified user we avoid add to cart and display a custom message
    elseif( $verified_user != 1 ) {
        $message = __( "Please, you need to be a verified user.", "woocommerce" );
        $button_text = __("proceed", "woocommerce");
        $url = home_url('/gld/my-account');
        $message .= ' <a href="'.$url.'" class="login-register button" style="float:right;">'.$button_text.'</a>';
        $passed = false; // Set to false
    }
    if( ! $passed )
        wc_add_notice( $message, 'error' );

    return $passed;
}

Code goes in function.php file of your active child theme (or active theme).

It should work.