how to create relational schema

To create a relational schema in Adobe Experience Platform (AEP):

1) Create two schemas: a Profile-enabled main schema and a non-Profile lookup schema.
2) Stitch them using the “Add Relationship” option by matching a common key (e.g., customerId).
3) Ingest data either:

Separately (recommended): Upload lookup and profile data in different dataflows.
Together: Use nested schema structure (less flexible and not reusable).

Referral Link: https://experienceleague.adobe.com/en/docs/experience-platform/xdm/tutorials/relationship-ui

I will suggest you to use separate lookup schemas with relationships for a cleaner, scalable, and reusable data model.