Automatically identify and retrieve users when logged into the application

I have a sign-up and login screen !. and wanted to know if you have how to sign up for the application if I will be able to get people's data in onClick without having to put id by id manually I need to put the id of the person manually! and I wanted to know if it has like the firebase to do this 'automatically' using the same child as this as "Usuários" and every click that the person gives in an attempt I can get the data and put in the same ID only with each .setValue which will be that of the person. I do not know if Realtime database can do this or will I have to use Cloudfire and so?