Search notes:

$ORACLE_HOME/rdbms/admin/dcore.bsq

dcore.bsq creates bootstrap tables (such as tab$ etc.) and other tables (such as dual etc…).
dcore.bsq is probably sourced/called from sql.bsq
A comment in doptim.bsq suggests that dcore.bsq handles pseudo-bootstrap objects.

Creating OBJ$

Interestingly, dcore.bsq creates the table obj$, but obj$ stores the name of all objects that are created. In fact, obj$ is the 18th object to be created (at least on Oracle 19c). 
select name from obj$ where obj# = 18;
This leaves one wondering how this and the previous records got written into obj$.

See also

dcore.bsq also creates the dual table.
$ORACLE_HOME/rdbms/admin

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