Show Custom Product By ID
First way
<?php
global $product;
$product = wc_get_product($productsgrp["product_1_id"]);//$productsgrp["product_1_id"]
echo apply_filters(
'woocommerce_loop_add_to_cart_link', // WPCS: XSS ok.
sprintf(
'<a href="%s" data-quantity="%s" data-product_id="'.$productsgrp["product_1_id"].'" rel="nofollow" class="%s" %s>%s</a>',
esc_url( $product->add_to_cart_url() ),
esc_attr( isset( $args['quantity'] ) ? $args['quantity'] : 1 ),
esc_attr( isset( $args['class'] ) ? $args['class'] : 'button product_type_subscription product_type_simple add_to_cart_button ord-butn ajax_add_to_cart mt-3' ),
isset( $args['attributes'] ) ? wc_implode_html_attributes( $args['attributes'] ) : '',
esc_html( $product->add_to_cart_text() )
), $product, $args);
?>
Second way
global $product;
echo do_shortcode( '[add_to_cart id=405]' );
Third way
$product = wc_get_product(get_the_ID());
echo "<a href='".$product->add_to_cart_url()."'>add to cart</a>";