These are built in classes and functions to handle some routine things.
Drupal/Component/Utility
https://api.drupal.org/api/drupal/namespace/Drupal!Component!Utility/
Utility classes you can use in your code.
This list is incomplete, check the link for more info.
Html
cleanCssIdentifier()
load($html): DOMDocument
decodeEntities()
Bytes
toNumber()
NestedArray
getValue()
/core/includes/
https://git.drupalcode.org/project/drupal/-/tree/11.x/core/includes
Useful functions.
This list is incomplete, check the link for more info.
common.inc
format_size() - human readable file size.