Firebase.firestore vs admin.firestore

I'm learning GCP and in their Firestore, I'm confused with the difference of Admin.firestore & Firebase.firestore.

this is the code for admin:

const admin = require("firebase-admin");
admin.initializeApp({
    credential: admin.credential.applicationDefault(),
    databaseURL: "https://<firestoreprojectnameurl>"
const db = admin.firestore();
  });

while this is the code for the firestore

const { config } = require('./config');
const firebase = require("firebase");
firebase.initializeApp(config);
const db = firebase.firestore();

Please note that only 1 db at a time will work and for my current set-up I use the db = firebase.firestore() although if I change it to db = admin.firestore ite works fine and all my code works the same.

Thank you in advance!