The temporary tablespace in Oracle is used to store data that is temporary and session specific (sort operations, hash joins, temporary tables data, etc.).
Users use the same temporary segments in the temporary tablespace to perform the sort operation, joins, etc. How can we tell which users use the temporary tablespace and how much space they use?

You can find information about usage of the temporary tablespace in the V$SORT_USAGE dictionary view. This view contains the following columns:

USERNAME – database user name

SESSION_ADDR – address of the session, can be used to identify the session in V$SESSION according to the SADDR column

SQL_ID – the identifier of the SQL that requires the sort or join, can be used to identify the SQL from V$SQL according to the SQL_ID column

EXTENTS – number of extents in the temporary segment being used by this session

BLOCKS – number of blocks in the temporary segment being used by this session