Read woocommerce search custom fields for more information.
Woocommerce Custom Fields
Custom fields are one of the more powerful features available in WordPress. They are particularly useful when extending WordPress via the use of custom post types. I create custom post types all the time for things like products, portfolios, or galleries when developing WordPress themes for clients. Custom fields are extremely handy when it comes to adding product details, such as item numbers or prices. Unfortunately, it’s not possible to search WordPress by custom fields out of the box. In order to fix that, we need to use a custom search plugin that will replace the default WordPress search.
Modifying the wp_query is one way to do this. But it is an advanced method and not totally necessary. To do this you will need to install and activate WP fastest site search plugin. It comes with a control panel where you can mention the custom fields to support Woocommerce search custom fields.