Checkout provides you parameters to checkout directly without clicking a single product page's "Add To Cart" button.
1) The parameter to add the product for checkout
Here is the parameter to add the product for checkout:
2) Sample URL
Here is the sample URL:
3) Where to get parameter
For example, here 87 is your product id in WooCommerce. You can find it by going to your product list and checking here:
4) Behavior in case of variations/variable
Similarly, If you want to add a particular variation of a variable product, you can directly put the Variation ID.
Find the variation ID by editing the product here:
This will add the product with pre-selected variation Blue color & XXL size for checkout
Here is the Sample URL:
This is how you can add a WooCommerce product with desired quantity directly to the Checkout Page using the URL/Link.