Where does the UID come from when implementing presence in Cloud Firestore

I searched for a long time on presence topic, and i found the following doc

https://firebase.google.com/docs/firestore/solutions/presence.

I followed the doc and made all the necessary settings , but there is something i did not understand it .

in previous link, Specifically on the .js code for Firebase function they written the following

// Create a new function which is triggered on changes to /status/{uid}
// Note: This is a Realtime Database trigger, *not* Firestore.
exports.onUserStatusChanged = functions.database.ref('/status/{uid}').onUpdate(
    async (change, context) => {...etc...}

i need to understand how they got the uid value in .ref('/status/{uid}')??

i know that status is parent for uid child but how they can access it's value within .ref('/status/{uid}')

I spent three hours reviewing the document, but I couldn't figure out how they got uid value

The first code snippet in the solution guide on using Cloud Functions and Realtime Database to build presence on Firestore shows the code that writes the presence information to the Realtime Database:

// Fetch the current user's ID from Firebase Authentication.
var uid = firebase.auth().currentUser.uid;

// Create a reference to this user's specific status node.
// This is where we will store data about being online/offline.
var userStatusDatabaseRef = firebase.database().ref('/status/' + uid);
...

So this code determines the UID of the user that is signed in, and then writes the presence information to the path /status/$uid in the Realtime Database. This then triggers the Cloud Function that you included, which picks the UID part of the path up here:

exports.onUserStatusChanged = functions.database.ref('/status/{uid}').onUpdate(
  ...

So this Cloud Function triggers whenever a user updates the status node, and takes the uid value that the client has written, so that is can write the presence information under the same UID to Firestore.