What is the @Factory annotation in TestNG and when do you use it?

// Test class with constructor parameter
public class LoginTest {
    private String username;
    private String password;
    private String expectedTitle;

    public LoginTest(String username, String password, String expectedTitle) {
        this.username = username;
        this.password = password;
        this.expectedTitle = expectedTitle;
    }

    @Test
    public void loginTest() {
        driver.findElement(By.id("username")).sendKeys(username);
        driver.findElement(By.id("password")).sendKeys(password);
        driver.findElement(By.id("loginBtn")).click();
        Assert.assertTrue(driver.getTitle().contains(expectedTitle));
    }

    @Test
    public void verifyProfileTest() {
        // Also uses this.username for profile verification
        driver.findElement(By.cssSelector(".profile-name"));
        Assert.assertTrue(/* profile visible */);
    }
}

public class LoginTestFactory {

    @Factory
    public Object[] createInstances() {
        return new Object[] {
            new LoginTest("admin",    "admin123",  "Dashboard"),
            new LoginTest("manager",  "mgr456",    "Reports"),
            new LoginTest("readonly", "read789",   "View Only"),
        };
    }
}

<suite name="MultiUserSuite">
    <test name="Login As Different Users">
        <classes>
            <class name="factory.LoginTestFactory"/>
            <!-- NOT LoginTest — factory creates those instances -->
        </classes>
    </test>
</suite>

public class BrowserFactory {

    @Factory(dataProvider = "browsers")
    public Object[] createBrowserInstances(String browser) {
        return new Object[] { new CrossBrowserTest(browser) };
    }

    @DataProvider(name = "browsers")
    public Object[][] browsers() {
        return new Object[][] {
            {"chrome"}, {"firefox"}, {"edge"}
        };
    }
}