After creating global attributes via the "Classes & Attributes" section (Catalog > Classes & Attributes), you need to assign these attributes to specific products.
Usually, all global attributes are automatically assigned to all products in your store (both existing and future ones) right after you create them. First, however, it's necessary to ensure you correctly configured the assigned global attributes to display in the storefront.
To assign and configure a global attribute for a product, follow the steps below:
In your store's Admin area, locate the product you need to assign a global attribute to, open the product details, and click on the "Attributes" tab.
Choose "Global" in the drop-down.
On the page that opens, scroll down to the "Global attributes" section and look at the list of attributes displayed in it.
Here you can see the complete list of your store's global attributes. The attributes for which the "Attribute option" field is empty are assigned but not configured for the product and are not displayed on the storefront for this purpose.
For the attribute that needs to be configured:
Click inside the "Attribute option" field and type in the value you require.
Click to expand the "Attribute option" drop-down and select a pre-configured attribute value from the list.
If you have more global attributes that need to be assigned to the product, repeat this for each of them.
Choose whether you want to apply the selected attribute option(s) to this product only or to extend it/them to all the products with the same global attribute by enabling the "Apply attribute value changes for all the products" option:
Click "Save changes."