https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21BubbleableMetadata.php/10
BubbleableMetadata.php | Drupal API
phpdrupalapi
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21BubbleableMetadata.php/function/BubbleableMetadata%3A%3Amerge/8.9.x
function BubbleableMetadata::merge | Drupal API
functionmergedrupalapi