The classic editor uses traditional AJAX actions that call custom PHP functions for database operations. Each action requires custom security checks, capability verification, and database handling code.
The block editor exclusively uses the WordPress REST API for all database interactions. Creating a category, for example, uses the standard WordPress categories endpoint rather than custom AJAX callbacks.
This REST API approach provides built-in security, standardized responses, and eliminates the need for custom AJAX handlers. However, it requires understanding REST API concepts rather than traditional WordPress hooks.