Create a startup trigger to start all PDBs after DB startup

SQL> create or replace trigger open_all_pdbs
  2    after startup on database
  3  begin
  4    execute immediate 'alter pluggable database all open';
  5  end;
  6  /

Trigger created.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area 1636814848 bytes
Fixed Size                  2288968 bytes
Variable Size             989856440 bytes
Database Buffers          637534208 bytes
Redo Buffers                7135232 bytes
Database mounted.
Database opened.
SQL> select name,open_mode from v$pdbs;

NAME                           OPEN_MODE
------------------------------ ----------
PDB$SEED                       READ ONLY
DEMO                           READ WRITE