User Experience is about users? users build the product?

If it includes user feedback, yes.  Many manufacturers proactively involve users to improve or "build" their product further.

There's a lot of applications, both mobile and desktop that are coming out. Most of them fail because it does not engage quality interaction with its end user.

Nowadays,  it's very important to understand how user's interact with your application, you can create a minimal to a highly creative design for a specific app, but the question is, is it effective? does it increase user engagement?, is it easy to use?

This kind of questions need tough research. In order to make your end product successful, you need to conduct human behavioral research on how they are going to use your product.