Where is the extension key specified in the composer.json of a TYPO3 extension?

TYPO3 reads the extension key from "extra": {"typo3/cms": {"extension-key": "my_ext"}} in composer.json. Since v14 this also applies in classic mode: an extension without a valid composer.json with a type starting with typo3-cms- and this entry is no longer detected. The package name in the name field (such as acme/my-ext) is a common misconception, because it follows Composer rules with a hyphen and a vendor and therefore often differs from the extension key with its underscore. The type field reads typo3-cms-extension for extensions and does not contain the key.