Where is the macos share extension implemented?

Hi all,
I am looking for ways to register an electron app with the macOS share extension menu in order to approach a longtime issue/feature request with the signal messenger. I just found out that my favorite mail program (much love) supports this functionality
Screenshot 2024-06-07 at 23.55.04
and itself is an electron application. However, I cannot find the slightest hint to how mailspring registers itself as a share extension (no swift code, no Info.plist mentions NSExtension, no Issue or commit mentioning it, I couldn’t even find the menu-item message from the screenshot). I am quite inexperienced in this regard, so excuse me if I overlooked something obvious. But if anyone knows how / where this functionality is achieved / implemented, I would be grateful to hear your wisdom!