Search notes:

$ORACLE_HOME/rdbms/admin/dsec.bsq

Security related data dictionary components.

Creation of user SYS

Among others, user SYS is created in dsec.bsq: 
create user sys identified by …
/
grant inherit any privileges to sys
/

Other intersing observations

The roles PUBLIC, CONNECT, RESOURCE, DBA (and others) are created in this script.
Tables table_privilege_map and system_privilege_map.

See also

$ORACLE_HOME/rdbms/admin

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