External ID is created on export if it doesn't exist already, you need to export products and check ( I want to update data (import-compatible export)) if you need to import them back to Odoo. Once you export, Odoo will export all selected products with selected fields and generate the external ID for all records which doesn't have one. Then you can update the fields you want to import them again to Odoo.

Another solution is to use Database ID (ID) rather than external ID.

Read more about external ID and Database ID here:

Export and import data — Odoo 15.0 documentation