Unfortunately, There is no default mechanism to start all Pluggable Databases.
While startup of Container Databases, all the Pluggable Databases will be in mounted state.
But we can accomplished same with the system trigger on Container Database to start PDB[s].

Following trigger will automatically open all PDB’s while starting up CDB.

CREATE OR REPLACE TRIGGER opn_pdbs
AFTER STARTUP ON DATABASE
BEGIN
EXECUTE IMMEDIATE ‘ALTER PLUGGABLE DATABASE ALL OPEN’;
END opn_pdbs;
/

We can customise it if we don’t want to start all PDB’s

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Note: Please don’t hesitate to revert in case of any query OR feedback.

Thanking you.

Have a easy life ahead.