CBD AND PDB

Within CDB

CDB_TABLESPACES 
note: you can see all tablespace belong to PDBs   

DBA_TABLESPACES 
note: you can see only tablespace belong to CDB 

Within PDB

CDB_TABLESPACES=DBA_TABLESPACES  
note you can see only tablespace belong to PDB  

SHOW commands 
show con_name

show con_id

V$CONTAINERS  = V$PDBS except in root container 
V$CONTAINERS
V$PDBS

CONNECT OR SWITCH AMONG CDB$ROOT, PDB$SEED or PDBs, 
show con_name
ALTER SESSION SET container =QA;
alter session set container=cdb$root;
alter session set container=pdb$seed;