This week's book giveaway is in the NodeJS forum. We're giving away four copies of Serverless Applications with Node.js and have Slobodan Stojanovic & Aleksandar Simovic on-line! See this thread for details.
Fist of all, what does the word "backendless" mean? It sounds like there might not be a backend, but obviously, there is - you talk of logging in somewhere, and storing data about users.
As to how to differentiate data amongst users - all DB tables containing data specific to a user would have the userID as a field. Or a foreign key into the user table, so that the connection between a record of data and a particular user is easy to establish.