I’m confused with the Oracle user access management model and it seems too complicated.

My case: I have a tenancy with few resources for personal use, so I’m the owner and the only user. I have two domains, “Default” and “OracleIdentityCloudService” with one user each (the account was set up long ago, so I don’t remember if they were created automatically or if I created the second one).

I don’t need two accounts (aka root and tenant) in different domains, I could use one.
Can I safely delete everything related to the OracleIdentityCloudService domain and the domain itself? Or is this some kind of standard domain used for tenants, and should I keep it?

The second question: In the “Identity > Domains > … > Security > Sign-on policies” menu, if a policy was customized, it has a “Restore defaults” button. But by looking at the policy, I can’t tell the difference compared to the default one. How can I see the difference for a Sign-on policy before restoring or keeping it?