Information about the current users of a database and their privileges is stored in the database system tables, which are accessible through system views.
Most system tables are owned by the SYS user ID. You cannot log in using the SYS user ID.
The DBA has SELECT access to all system tables, just as to any other tables in the database. The access of other users to some of the tables is limited. For example, only the DBA has access to the SYS.SYSUSERPERM table, which contains all information about the privileges of users of the database, as well as the passwords of each user ID. However, SYS.SYSUSERPERMS is a view that contains all information in SYS.SYSUSERPERM except passwords, and by default, all users have SELECT access to this view. All privileges and role memberships that are automatically set up in a new database for SYS and PUBLIC system roles, and DBA user can be fully modified.