The as400 is nowhere near a "little brother" to os390 or z_os. The as400 is largely operated by clerks rather than data processing professionals. They are often/usually installed "turn-key" and do not have a local technical support staff - they are primarily "user" systems rather than development systems (at least the several hundred i've seen).
The os390/z_os world has far more capability and is almost always supported by multiple admin people. Someone is responsible for db2 on the system you are using. Whoever that is, can get a list of tables that you have permission to use. It is not complicated, it is just not the same as the as400 environment.
Hope this helps,