Your question is not very specific, and I do not know what you mean by the “default tables”. but let me try to address the topic.
DB2 z/OS consists of many datasets. Each tablespace and each index space will be made up of one (or more) VSAM LINEAR datasets. The naming convention for these datasets is as follows:
high level qualifier . DSNDBx . data base name . space name . I0001. A001
high level qualifier depends on your installation and VCAT name, and DB2 STOGRP definitions
DSNDBx is DSNDBC for cluster component and DSNDBD for the data component
database name is the name of the database in question (DSNDB01 is the DB2 directory, DSNDB06 is the catalog, DSNDB04, DSNDB07, and so on)
space name is the name of the tablespace or index space
the last two nodes of the dataset name (starting with I & A) are used by in line REORG and for multiple datasets per page space (which is too involved to get into right now).
Now, please do not try to access the datasets directly. DB2 only wants you to access the data via the table structures. Only your DBA should EVER consider accessing the datasets directly.
I hope this helps.