Safari app extensions and Safari web extensions are both tools that allow developers to extend certain features and functionalities of the Safari browser. However, there is a key difference between them:
Safari app extensions are built using native technologies such as Swift or Objective-C for macOS apps, while Safari web extensions use HTML, CSS, JavaScript and other web technologies. This means that a developer can create an extension in either format depending on their specific needs.
For example, if they need access to system-level APIs or want to add a custom user interface element then they would build it as an app extension; however if they just wanted to modify existing UI elements or change behavior within the browser itself then it would be better suited as a web extension.