Here’s how you would align these buttons if you’re using default WooCommerce styles. You could adjust the heights here as needed, and add this CSS to your child-theme stylesheet:

a.added_to_cart.wc-forward {
bottom: 0;
position: absolute;
display: block;
bottom: 0px;
}

.woocommerce ul.products li.product,
.woocommerce-page ul.products li.product {
min-height: 295px !important; /* <- modify the height of the gap here */
margin-bottom:10px;
}

a.button.add_to_cart_button,
a.button.product_type_simple {
bottom: 30px;
position: absolute;
}

 
Source: https://www.skyverge.com/blog/woocommerce-align-add-to-cart-buttons/