Unity Catalog is an operational metastore. That means as the metadata/data is changing the changes are reflected in the UC in real time. Whereas, think of Purview as an Enterprise catalog where metadata is fed from UC or many other database systems used by the organization.

There's some changes to Purview coming out that enable users to create Data Products in Purview as the start of a Data Fabric which is realized in Databricks. ER/Studio allows you to design structures with well-formed data models of your Databricks Lakehouse which can be published back to Purview.