I found this file
and if I modify this file by adding something like:
* * * * * date >>/tmp/cron-date.log
then I can see the contents using crontab -l and also it writes something in the tmp folder so it works.
However it still seems that I crontab -e is not modifying this file, editing with crontab -e does not do anything at the end.
DId I miss anything here ?