How does Truecaller work

When you install Truecaller App in your mobile or when you signup in Truecaller, it asks for some permission from you. One of these permissions is Contact Access, when you allow this permission, then Truecaller is able to see all the contacts in your phone and also saves them in its server.

With this, all your names and numbers are saved with Truecaller. Now in Truecaller, you search for a number or you are getting a call from someone, then Truecaller searches that number in its server. And the name by which that number is saved in the server, that name appears in your Incoming Call or Search Result.