The business logic and data access code is contained in the backend project.
The UI is implemented with Vaadin, Hilla and Thymeleaf and each technology has its own project.
The Sakila sample database was initially developed by Mike Hillyer, a former member of the MySQL AB documentation team. It is intended to provide a standard schema that can be used for examples in books, tutorials, articles, samples, and so forth. The Sakila sample database also serves to highlight features of MySQL such as Views, Stored Procedures, and Triggers.
Source: https://dev.mysql.com/doc/sakila/en/sakila-introduction.html
This example uses a PostgreSQL version from https://github.com/jOOQ/sakila