Declarative schema is the new feature in Magento to do database operations of creating and updating database tables. This is one of the major changes introduced by the Magento team in 2.3 and it replaces the old approach of creating InstallSchema & UpgradeSchema scripts. Why Declarative Schema? Before declarative schema, developers need to write different schema scripts in each version of a module. And when a customer upgrades Magento to a version several releases ahead of the installed version, the upgrade scripts between those two versions will still be executed. This increases the complexity to track changes with every upgrade script. And new declarative schema approach allows developers to declare the final desired state of the database and has the system adjust to it automatically. What is the use of db_schema.xml? Before Magento 2.3, whatever database operations developers were writing in InstallSchema & UpgradeSchema scripts are replaced by the db_schema.xml file. So now...