The getMessaging() function that you are referring to is a top level function that can be imported from firebase-admin/messaging SDK as shown below:
import { getMessaging } from "firebase-admin/messaging";
This is equivalent of admin.messaging() and both return an instance of Messaging class.
If you are following Firebase documentation to send messages, the code should be:
import { getMessaging } from "firebase-admin/messaging"; // not just "firebase-admin"
const messaging = getMessaging();
await messaging.send(...)
The Admin SDK is not fully modular yet like the the client SDK so rest of the syntax remains same. Also checkout What is the difference between admin.firestore, admin.firestore() and getFirestore()