How debug logs are set in Salesforce?

To activate debug logging for users, Apex classes, and Apex triggers, configure trace flags and debug levels in the Developer Console or in Setup. Each trace flag includes a debug level, start time, end time, and log type. The trace flag’s log type specifies the entity you’re tracing.
You can retain and manage debug logs for specific users, including yourself, and for classes and triggers. Setting class and trigger trace flags doesn’t cause logs to be generated or saved. Class and trigger trace flags override other logging levels, including logging levels set by user trace flags, but they don’t cause logging to occur. If logging is enabled when classes or triggers execute, logs are generated at the time of execution.
Debug logs have the following limits.
Each debug log must be 20 MB or smaller. Debug logs that are larger than 20 MB are reduced in size by removing older log lines, such as log lines for earlier System.debug statements. The log lines can be removed from any location, not just the start of the debug log.
System debug logs are retained for 24 hours. Monitoring debug logs are retained for seven days.
If you generate more than 1,000 MB of debug logs in a 15-minute window, your trace flags are disabled. We send an email to the users who last modified the trace flags, informing them that they can re-enable the trace flag in 15 minutes.
When your org accumulates more than 1,000 MB of debug logs, we prevent users in the org from adding or editing trace flags. To add or edit trace flags so that you can generate more logs after you reach the limit, delete some debug logs.