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SMON

Starting the database

SMON checks the SCN in all datafile headers when the database is started. Everything is OK if all of these SCNs matches the SCN found in the controlfile. If the SCNs don't match, the database is in an inconsistent state.
SMON also frees temporary segments.

smon_scn_time

Every 5 minutes, SMON wakes up and fills sys.smon_scn_time

Misc

SMON also updates sys.col_usage$ (if _column_tracking_level is set to 1)