WooCommerce sku location in DB

As suggested in comments, the best way to update stock, or any other product information would be through the WooCommerce API.

The product endpoint with all the parameters is documented here (link is directly to how to update): https://woocommerce.github.io/woocommerce-rest-api-docs/#update-a-product.

I’m not very familiar with woocommerce, but based on other posts it looks like the stock value is stored in the _stock key in wp_postmeta, see here for more info on that and how to e.g. convert between post ID and SKU if you need to. Again, note that not doing this through the API you might miss out on other code that gets triggered if WooCommerce would usually do other things (e.g. if stock is set to zero maybe it updates other fields automatically or triggers other events if you update through the API)