How to pass/get data to/from the WooCommerce data-product_variations object?

  jQuery(document).on('found_variation.wc-variation-form', 'form.variations_form', function(event, variation_data) {
    //this is called when a valid productis found
  });
  jQuery(document).on('change.wc-variation-form', 'form.variations_form', function(event) {
    //this function is called when the user clicks or changes the dropdown
  });

The PHP function you are looking for is

apply_filters(
        'woocommerce_available_variation',
        array(
            'attributes'            => $variation->get_variation_attributes(),
            'availability_html'     => wc_get_stock_html( $variation ),
            'backorders_allowed'    => $variation->backorders_allowed(),
            'dimensions'            => $variation->get_dimensions( false ),
            'dimensions_html'       => wc_format_dimensions( $variation->get_dimensions( false ) ),
            'display_price'         => wc_get_price_to_display( $variation ),
            'display_regular_price' => wc_get_price_to_display( $variation, array( 'price' => $variation->get_regular_price() ) ),
            'image'                 => wc_get_product_attachment_props( $variation->get_image_id() ),
            'image_id'              => $variation->get_image_id(),
            'is_downloadable'       => $variation->is_downloadable(),
            'is_in_stock'           => $variation->is_in_stock(),
            'is_purchasable'        => $variation->is_purchasable(),
            'is_sold_individually'  => $variation->is_sold_individually() ? 'yes' : 'no',
            'is_virtual'            => $variation->is_virtual(),
            'max_qty'               => 0 < $variation->get_max_purchase_quantity() ? $variation->get_max_purchase_quantity() : '',
            'min_qty'               => $variation->get_min_purchase_quantity(),
            'price_html'            => $show_variation_price ? '<span class="price">' . $variation->get_price_html() . '</span>' : '',
            'sku'                   => $variation->get_sku(),
            'variation_description' => wc_format_content( $variation->get_description() ),
            'variation_id'          => $variation->get_id(),
            'variation_is_active'   => $variation->variation_is_active(),
            'variation_is_visible'  => $variation->variation_is_visible(),
            'weight'                => $variation->get_weight(),
            'weight_html'           => wc_format_weight( $variation->get_weight() ),
        ),
        $this,
        $variation
    );

This is found here https://github.com/woocommerce/woocommerce/blob/master/includes/class-wc-product-variable.php#L325

WC_Product_Variable -> get_available_variation( $variation )