CIBIL or other credit bureaus are repository of information, where by they store and disseminate information about credit behaviour of borrowers.
While they may seem like God to know a lot about you (just take a look at what all your CIBIL report contains) and affecting you in your financial matters, the information contained in the bureaus is all based on data submitted by banks and other such institutions.
Every month or even more frequently banks submit information about your loans to CIBIL and other credit bureaus. In this data submission not only details of your loans like timely repayment and amount outstanding are shared but information about address, phone numbers etc also shared. Apart from this when you apply for a new loan then the bank enquiries about you with the bureau to find out your details.
At that time the information submitted to the bureau like name, DOB, address and phone numbers along with loan type (Personal loan, credit card etc.) and amount of the loan you applied for are also saved by the bureau and updated to your profile.