You would have probably come across this while copying code off of a website, or an API key, or copying links from Google Drive:
The clipboard API
document.execCommand() has been available to copy text way before the clipboard API became a thing. But it was a synchronous call, did not work correctly across all browsers (permission access was not consistent either), and had some security risks associated with it.
The newer asynchronous clipboard API is supported by all browsers and is more secure (only works on HTTPS pages by default, and is not available to background tabs).
And for copying from it:
We will need more code to detect browser support and error handling. But that is the gist of the API.
Both methods are supported in all modern browsers. And that is it. I hope this was helpful and that you start using it in the relevant places.