InShare | Simple file sharing web app with drag & drop | File Sharing library

Hey Respectful Programmers! I am making a project of voice recording application (with cloud storage functioning) and In this application, the audio gets saved in the phone storage in a folder "Zrecorder" (my app name) and inside this, there is a folder created named the userName's email (example - vijay@gmail.com) and then the audio is inside the folder in mp3 format. When the user backup the data then the folder is uploaded to firebase storage only if the user is logged in already. Now in firebase storage, there is a folder created the same as we created in phone storage in the same way. So when the user restores data, first we get their userName and then we find the userName folder in firebase storage then we download the folder in the phone storage (folders cannot be downloaded so I used loops to get links of each file and download them to a specific folder by download manager class). I hope you understand what I am saying.

So here is my problem: I want that each user should have access to their own folder only. I saw that on firebase docs but I did not understand the code properly. They say do it using 'uid' but I don't understand that.

This is how user login into my app.