Hi,
Those data which is created from data files/demo files will have external id. If you need to generate an external id for the record created from the user interface, open the list view of the model, select the record you need to generate external id and export it using import compatible format.

Once you trigger export, odoo will generate and set an external id for the record.

See:  How To Generate External ID For The Records Created From User Interface Odoo

Thanks