Which one is created first? Database or Instance?
Introduction to the Oracle Database Instance

A database instance is a set of memory structures that manage database
files. A database is a set of physical files on disk created by the
CREATE DATABASE statement. The instance manages its associated data
and serves the users of the database.
Every running Oracle database is associated with at least one Oracle
database instance. Because an instance exists in memory and a database
exists on disk, an instance can exist without a database and a
database can exist without an instance.

A database can exist without an instance, but to use that database, you need to start an instance, then mount and open the database in it.
You can &quot;create&quot; a database by simply copying the files of an existing database. You do not need an instance for that. But to use that database, you need an instance.
If you want to create a database from scratch, you need a running instance first.