Explanation/Reference:

The alert log is a chronological log of messages and errors, and includes the following items:
*All internal errors (ORA-600), block corruption errors (ORA-1578), and deadlock errors (ORA-60) that occur
*Administrative operations, such as CREATE, ALTER, and DROP statements and STARTUP, SHUTDOWN, and ARCHIVELOG statements
*Messages and errors relating to the functions of shared server and dispatcher processes
*Errors occurring during the automatic refresh of a materialized view
*The values of all initialization parameters that had nondefault values at the time the database and instance start Note:
*The alert log file (also referred to as the ALERT.LOG) is a chronological log of messages and errors written out by an Oracle Database. Typical messages found in this file is: database startup, shutdown, log switches, space errors, etc.
This file should constantly be monitored to detect
unexpected messages and corruptions.

Explanation/Reference:
Explanation:
ENABLE_DDL_LOGGING enables or disables the writing of a subset of data definition language (DDL) statements to a DDL alert log.
The DDL log is a file that has the same format and basic behavior as the alert log, but it only contains the DDL statements issued by the database. The DDL log is created only for the RDBMS component and only if the ENABLE_DDL_LOGGING initialization parameter is set to true. When this parameter is set to false, DDL statements are not included in any log.

Note: * In a container architecture, the parameters for PDB will inherit from the root database. That means if statistics_level=all in the root that will cascade to the PDB databases.
You can over ride this by using Alter system set, if that parameter is pdb modifiable, there is a new column in v$system_parameter for the same.