External lookup relationship links a child standard, custom, or external object to a parent external object. The values of the standard External ID field on the parent external object are matched against the values of the external lookup relationship field. For a child external object, the values of the external lookup relationship field come from the specified External Column Name.

In External lookup relationship, External Object will be the parent.

Indirect lookup relationship

Indirect lookup relationship links a child external object to a parent standard or custom object. You select a custom unique, external ID field on the parent object to match against the child’s indirect lookup relationship field, whose values are determined by the specified External Column Name.

In Indirect lookup relationship, Salesforce standard or custom object will be the parent and External Object will be the child.