Confirm which container you are in and query all PDBs from the root (CDB_ / CONTAINERS)

Command

-- Current container:
SHOW CON_NAME
SHOW CON_ID
SHOW PDBS
SELECT SYS_CONTEXT('USERENV','CON_NAME') AS con_name,
       SYS_CONTEXT('USERENV','CON_ID')   AS con_id
FROM   dual;

-- All containers, from the root:
SELECT con_id, name, open_mode FROM v$containers ORDER BY con_id;

-- CDB_ views combine all open PDBs with CON_ID:
SELECT c.name AS container, t.tablespace_name, t.status
FROM   cdb_tablespaces t JOIN v$containers c ON c.con_id = t.con_id
ORDER  BY c.name, t.tablespace_name;

-- Query any table/view across all PDBs with CONTAINERS():
SELECT con_id, COUNT(*) AS users
FROM   CONTAINERS(dba_users)
GROUP  BY con_id ORDER BY con_id;

What it does

Shows the container the session is connected to; from the root, CDB_ views and the CONTAINERS() clause collect dictionary information from all open PDBs in a single query.

When to use it

To make sure you are in the right container before a DDL/parameter change, and to build a space, user and object inventory across all PDBs.

Caution

Read-only. Do not accidentally create application objects or local users in the root (CDB$ROOT); ALTER SYSTEM in the root affects the whole CDB. CDB_ views and CONTAINERS() return only open PDBs; MOUNTED PDBs do not appear in the result.