Oracle - How does Oracle manage transaction specific DML statements

Question

Imagine I have this simple table:

Table Name: Table1
Columns:    Col1 NUMBER (Primary Key)
            Col2 NUMBER

If I insert a record into Ta

Answer 1

It's because of the unique index that enforces the primary key constraint. Even though the insert into the data block is not yet committed, the attempt to add the duplicate entry into the index cannot succeed, even if it's done in another session.