https://mikedietrichde.com/2016/10/21/gather-fixed-objects-stats-in-pdbs-as-well/#:~:text=Yes%2C%20you'll%20have%20to,independently%20from%20the%20CDB%24ROOT.&text=Oracle%20Database%20automatically%20gathers%20fixed,Automatic%20Optimizer%20Statistics%20Collection%E2%80%9C) .
Reference:
https://docs.oracle.com/en/database/oracle/oracle-database/20/arpls/DBMS_STATS.html

Except for the tablespace space usage metric, which is database related, the other metrics are instance related. Threshold alerts are also referred to as stateful alerts which are automatically cleared when an alert condition clears. Stateful alert appears in DBA_OUTSTANDING_ALERTS and when cleared go to DBA_ALERT_HISTORY. Other server-generated alerts correspond to specific database events such as ORA-* errors, "Snapshot too old" errors, Recovery Area Low on Free Space, Resumable Session Suspended. These are non threshold based alerts, also referred to as stateless alerts. Stateless alerts go directly to the History table. +++ Most alerts (such as "Out of Space") are cleared automatically when the cause of the problem disappears. However, other alerts (such as generic alert log errors) are sent to you for notification and must be acknowledged by you. After taking the corrective measures, you acknowledge an alert by clearing or purging it. Clearing an alert sends the alert to the Alert History which is accessible from Monitoring sub menu. Purging an alert removes it from the Alert History.
Reference:
https://jameshuangsj.wordpress.com/2019/12/01/clears-stateless-alerts-in-oem-by-using-emcli/