I’m reading the documentation of Extend Cloud Firestore with Cloud Functions.

The code is as follows

// Listen for changes in all documents in the 'users' collection
exports.useWildcard = functions.firestore
    .document('users/{userId}')
    .onWrite((change, context) => {
      // If we set `/users/marie` to {name: "Marie"} then
      // context.params.userId == "marie"
      // ... and ...
      // change.after.data() == {name: "Marie"}
    });

In functions.firestore.document('users/{userId}'), I don’t really understand the {userId}.
I know every collection in firestore has a document ID. I have saved my document ID in the field of every user as UID. So, do I replace the {userId} with my UID or I should just use the {userId} as it it or do I need to use the specific field I want to trigger?
In my case I want to trigger the field time in my document.