Oracle 21c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
ALTER SESSION SET NLS_TIMESTAMP_FORMAT = 'YYYY-MM-DD HH24:MI:SS.FF';
select '1' || '2' from dual;

'1'||'2'

12

select 1 + 2 from dual;

1+2

3

select '1' + '2' from dual;

'1'+'2'

3

select TO_NUMBER('1') + TO_NUMBER('2') from dual;

TO_NUMBER('1')+TO_NUMBER('2')

3