FFirst it is important to understand, that when you build your custom login ui, it is not neccessarly required to also have the oidc standard.
FSo you can find different guides on how to build the login ui, and the different flows here: https://zitadel.com/docs/guides/integrate/login-ui
In the following guides you will learn how to create your own login UI with our APIs. The different scenarios like username/password, external identity provider, etc. will be shown.
Fif you do need the oidc standard for your login ui, you need to implement this in your login. we do have a guide here on how the oidc standard can be implemented: https://zitadel.com/docs/guides/integrate/login-ui/oidc-standard
To build your own login ui for your own application it is not necessary to have the OIDC standard included or any additional work that has to be done.