Thanks 
Gloomie
Yes, I used a plugin by Sparsha: https://sparsha-dhar.itch.io/construct-3-firebase-plugin
There are two plugins ($12), one manages user accounts, and second handles Real-time Database, both through Firebase.
It's really simple to set-up, he's got well documented guide and also has a Discord channel for chat (help). Once you've created a Firebase account, and added your project there. The C3 side is as simple as using any other C3 built-in plugin. Straight-forward condtions/actions, e.g. Sign up, Sign in, Read-Data, Write-Data, Read Leaderboard, Incremeent Score value.. etc.
Real-time database in Firebase is made extremely simple to use, and looks like this:
So without any extra web/programming know-how, you can easily work with user-accounts and store/fetch user data, both: on C3 side and Firebase console.