Agile Requirements confusions

Hello

I am studying agile scrum methodology for software development. Coming from a waterfall background, I found something confusing.

Suppose, I am building an application that requires user authentication. So the user story will be like this:

"As a User, I want to register to the website using my mobile number and password so that I can use the system"

Now the problem is, there will be some sort of functionalities behind this single user story. For example, OTP should be sent to user mobile number for verification is a sub-requirement. This thing is not written in the user story.

My question is, how do I define/formalize these kind of background requirements in Agile Scrum? What are these things called in Agile? And where should I define them?

Thanks