In Linux, Oracle is owned by the oracle Linux account and that has an associated .bash_profile that is applied to every Oracle user log in. In that profile, the environment variables are set, ORACLE_HOME, ORACLE_BASE, ORACLE_SID, LD_LIBRARY_PATH, and PATH, typically.
Windows doesn’t explicitly do this. You log in as Administrator, and if Oracle has been installed, it is implied that it is the default database. Given no help at all, SQLPlus will manufacture a connection string based on defaults…the default for the host is localhost, the default for the port is 1521, and the default for Oracle SID is ORCL. Usually Oracle will mine through the registry for the SID. In the event that this is indeterminate, then it goes for the defaults.
ORACLE_HOME and ORACLE_SID among other settings should be created in the Windows registry by the installer.