Relationship b/w external object and custom or standard object in salesforce?

External objects support standard lookup relationships, which use the 18-character Salesforce record IDs to associate related records with each other. However, data that’s stored outside your Salesforce org often doesn’t contain those record IDs. Therefore, two special types of lookup relationships are available for external objects: external lookups and indirect lookups.
External lookups and indirect lookups compare a specific field’s values on the parent object to the relationship field’s values on the child object. When values match, the records are related to each other.
To create an external object relationship, create a custom field on the child object with one of the following field types. If the child is an external object, you can instead change the field type of an existing custom field to one of the following.
Lookup Relationship
External Lookup Relationship
Indirect Lookup Relationship