What is this inter-app communcation hook/API used to receive iOS Notes content when it is shared? (screenshot)

I'm wading through the documentation and finding lots of different things about inter-app communication and sending files around and starting apps from other apps, but I haven't found any screenshots or sample results in the documentation. I'm getting overwhelmed and I just want to know what is actually being used to accomplish the thing in the following screenshot:

the overlay that opens when you click the little arrow up box in the upper right corner of the Notes app

Once you click one of the app options in that overlay, another screen opens that contains the text from the Note file, and it is a different screen depending on your choice. If you click Messages, it prepares to send a text. If you click Messenger it prepares to send a message, etc.

It SEEMS like this is just about registering a corresponding file type, but I'm not sure what file type a Notes file really is or if that is what is actually going on at this point. To be clear, I'm asking: what is it that Messages, Mail, Slack, and Messenger do in their apps to make them appear in that list and respond when they are chosen?