What are dynamic tests in JUnit 6 and how does @TestFactory work?

import org.junit.jupiter.api.DynamicTest;
import org.junit.jupiter.api.TestFactory;
import static org.junit.jupiter.api.DynamicTest.dynamicTest;

class DynamicTestDemo {

    // @TestFactory: generates tests at runtime from a Stream
    @TestFactory
    Stream<DynamicTest> dynamicTestsFromStream() {
        return Stream.of("apple", "banana", "cherry")
            .map(fruit ->
                dynamicTest(
                    "Is non-empty: " + fruit,
                    () -> assertFalse(fruit.isEmpty())
                )
            );
    }

    // Generate tests from external data source (database, file)
    @TestFactory
    Stream<DynamicTest> testsFromDatabase() {
        return testDataRepository.findAll().stream()
            .map(scenario -> dynamicTest(
                scenario.getName(),
                () -> {
                    Result result = service.process(scenario.getInput());
                    assertEquals(scenario.getExpected(), result);
                }
            ));
    }

    // DynamicContainer: nested dynamic structure
    @TestFactory
    Stream<DynamicNode> nestedDynamic() {
        return Stream.of("cats", "dogs")
            .map(category -> DynamicContainer.dynamicContainer(
                "Tests for " + category,
                Stream.of(
                    dynamicTest("has species", () -> assertNotNull(category)),
                    dynamicTest("is not empty", () -> assertFalse(category.isEmpty()))
                )
            ));
    }
}