Oracle 12.2 and Oracle 19.6 - Container Database START/SHUTDOWN :
SQL*Plus: Release 19.0.0.0.0 - Production on Sat Dec 26 20:32:30 2020
Version 19.6.0.0.0

SQL> startup nomount;
ORACLE instance started.
Total System Global Area 5368705960 bytes
Fixed Size                  9159592 bytes
Variable Size            2315255808 bytes
Database Buffers         2936012800 bytes
Redo Buffers              108277760 bytes

CON_NAME
------------------------------
CDB$ROOT

SQL> select name,open_mode,con_id,dbid from v$containers;
no rows selected

No CDB or PDBs info at NOMOUNT state. NOMOUNT use for re-create or restore a missing control file for the CDB instance.

SQL> alter database mount;
Database altered.

SQL> select name,open_mode,cdb from v$database;
NAME                           OPEN_MODE            CDB
------------------------------ -------------------- ---
CJTED                          MOUNTED              YES

SQL> SELECT con_id, name, open_mode FROM v$pdbs;
    CON_ID NAME                           OPEN_MODE
---------- ------------------------------ ----------
         2 PDB$SEED                   MOUNTED
         3 PJTED1                         MOUNTED
         4 PJTED2                         MOUNTED

CDB’s control files opened for the instance, but both the CDB$ROOT and all PDBs are changed to the MOUNT state.

SQL> alter database open; or alter database CJTED open;
Database altered.

SQL> select name,open_mode,con_id,dbid from v$containers;
NAME                           OPEN_MODE      CON_ID       DBID
------------------------------ ---------- ---------- ----------
CDB$ROOT                  READ WRITE          1  105692156
PDB$SEED                   READ ONLY           2  657829016
PJTED1                         READ WRITE          3 1002662416         >>> Oracle 12.2   
PJTED2                         READ WRITE          4  531562146

SQL> SELECT con_name, instance_name, state FROM dba_pdb_saved_states;   >>> Oracle 12.2
no rows selected

CDB$ROOT is OPEN, it’s available for READ and WRITE operations. 
All PDBs are still READ WRITE with the seed database PDB$SEED mounted as READ ONLY:
dba_pdb_saved_states is valid for Oracle 12.1

========================= shutdown ================================

SQL> ALTER PLUGGABLE DATABASE ALL CLOSE IMMEDIATE;
Pluggable database altered.

SQL> select name,open_mode,con_id,dbid from v$containers;
NAME                           OPEN_MODE      CON_ID       DBID
------------------------------ ---------- ---------- ----------
CDB$ROOT                      READ WRITE          1  105692156      >>>> ROOT
PDB$SEED                       READ ONLY           2  657829016       >>>> SEED
PJTED1                            MOUNTED             3 1002662416
PJTED2                            MOUNTED             4  531562146
CSTG1                             MOUNTED             5 2604502831

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

It will shutdown ALL PDBs, PDB$SEED & CDB$ROOT inside CDB.