Questions related to integrating Zitadel with my application

I'm struggling to understand something related to using an identity platform like Zitadel with my application. For example, I often see websites that the user can register, login, reset credentials, has a profile page, can see the sessions and everything seems to be in the same application, nothing external like I often see with identity platforms that has the login page and everything separated that I must redirect the user. I understand this can be personalized and a few often has UI elements that we can use to make it feel like the user is not doing something externally but what I'm mostly struggling with is in the backend.

Identity platforms has their own internal tables and states to handle things but my application also needs the user table and other information for relations and the identity platforms often gets that part of handling most things about the user and that is what makes me confused. If the identity platform is handling the user table, how am I going to internally connect that user with other things like posts (for example)? How do normally applications that use identity platforms handle this situation? I have to duplicate things like have an user registered in the identity plaform and one in my application and sync things up? Or there's a better way of doing that and I'm missing out?

Thank you