Several parts which are installed on my system aren't shown in the "Embedding" tab of the dialog for the corresponding mime types. For example, for the "inode/directory" mimetype, I see the Konqueror sidebar and Konsole part but not Dolphin part. For "application/pdf" there's nothing listed at all, despite Okular part being installed and working correctly.
Parts which aren't listed can still work, but there's no way to manipulate, for example, the preference order.

I think this is related to the switching from .desktop files for parts to json-based plugins (see, for example https://invent.kde.org/system/dolphin/-/commit/1618a3ed21761648b20a14d7477aa065a2239fa7 for Dolphin).