We usually create the database during the installation of Oracle Database Software, however you can create the database after installing Oracle for several reasons:

If you used Oracle Universal Installer (OUI) to install the Software only and did not create the database.

Create another database on the same server.

If you want to make a copy of the existing database (Clone).

You can create the database in the following ways:

Database Configuration Assistant (DBCA) tool.

By CREATE DATABASE SQL statement.