[1] It is stored in two locations: data dictionary tables (accessed via built-in views) and a metadata registry.
Other relational database management systems support an ANSI-standard equivalent called information schema.
The total number of these views depends on the Oracle version, but is in a 1000 range.
Naturally the access to "DBA_" metadata views requires specific privileges.
Find view columns Warning: This is incomplete with respect to multiple datatypes including char, varchar and timestamp and uses extremely old, deprecated dictionary views, back to oracle 5.