How do you handle iframes in Selenium WebDriver?

// Switch to first iframe on page (0-based index)
driver.switchTo().frame(0);

// Interact with element inside iframe
driver.findElement(By.id("username")).sendKeys("testuser");

// Switch back to main page
driver.switchTo().defaultContent();

// <iframe name="loginFrame" id="loginFrame">
driver.switchTo().frame("loginFrame");
driver.findElement(By.name("password")).sendKeys("secret");
driver.switchTo().defaultContent();

WebElement iframeElement = driver.findElement(By.cssSelector("iframe.content-frame"));
driver.switchTo().frame(iframeElement);

driver.findElement(By.id("submitBtn")).click();
driver.switchTo().defaultContent();

// Switch to outer iframe
driver.switchTo().frame("outerFrame");

// Then switch to inner iframe within it
driver.switchTo().frame("innerFrame");

driver.findElement(By.id("element")).click();

// Go up one level (to outerFrame)
driver.switchTo().parentFrame();

// Go back to main page
driver.switchTo().defaultContent();

WebDriverWait wait = new WebDriverWait(driver, Duration.ofSeconds(10));
wait.until(ExpectedConditions.frameToBeAvailableAndSwitchToIt(
    By.cssSelector("iframe#content")
));
// Now inside the iframe
driver.findElement(By.id("element")).click();

// BAD — trying to find element inside iframe without switching
driver.findElement(By.id("insideIframe")); // NoSuchElementException

// GOOD
driver.switchTo().frame("myFrame");
driver.findElement(By.id("insideIframe")); // works
driver.switchTo().defaultContent();