How should an Architect create a relationship between the Invoice object and the Invoice Line Item object to ensure all users see all line items?

Creating a Master-Detail relationship between the Invoice Line Item object and the Invoice object ensures that all line items are associated with a specific invoice. In this relationship type, the detail (in this case, Invoice Line Item) inherits the sharing and visibility settings from the master (Invoice). This means that when users have access to a specific invoice, they will automatically have access to all associated line items, allowing them to see all relevant data seamlessly.

In contrast, using a Lookup relationship would not enforce the visibility rules as strictly; users might have access to the Invoice object without necessarily being able to see related line items, depending on their permissions. Master-Detail relationships also enable roll-up summary fields on the master object, providing additional reporting capabilities that wouldn't be possible with a simple Lookup relationship.

An inner join is not applicable in this context, as it's more related to querying data in a database rather than defining relationships in Salesforce. Therefore, establishing a Master-Detail relationship by placing it on the Invoice Line Item object while referencing the Invoice object is the correct approach to ensure visibility of all line items for all users.