The default logging configuration for Azure Web Apps in an environment in DXP has the following settings:

Application logging is turned on, with level Error, and logs are stored in the BLOB storage with 90 days retention.
Web server logging is turned on, and logs are stored in the BLOB storage with 90 days retention.
Detailed error messages are turned off.
Failed request tracing is turned off.

You can access application logs for environments from the DXP self-service as a live stream or download them for offline analysis.

The configuration above is applied to environments during provisioning, and the customer can change it to the Integration environment. Contact Support to change any of the following for Preproduction or Production environments:

Get access to web server logs.
Temporarily change the logging level.
Temporarily enable detailed error messages.
Temporarily enable failed request tracing.

You should use the Optimizely logging abstraction described in Log into .Net Diagnostics Trace.

In a DXP environment, the logger factory is set to EPiServer.Logging.TraceLoggerFactory, so application log messages are stored in the BLOB storage.