Glossary
Lookup column
A lookup column shows a value from another SharePoint list, so an order can display its customer's name. It displays a value; it is not a database join and it enforces no referential integrity.
That distinction is where projects get hurt. Deleting the customer will not stop you leaving orphaned orders behind, because nothing is enforcing the relationship.
Lookups also get slow, and they count against the view threshold in their own way. If your data genuinely needs enforced relationships, that is one of the few honest reasons to move to Dataverse.