What configuration involved in redirect to www domain from without www

Try to be thorough because this is so much trouble when someone else left and new IT comes, then just could not figure out why this domain without www is kept being redirected ( from the server side, because nothing to see from developer toolbar) to the www domain.

I have fair amount of know-how about Apache and .htaccess. I check logs. Yes, the request hit the log. And browser cache, yes, I clean them up. Try incognito mode etc.The redirect still goes on.

I tried to grep the word www under /etc/apache2, nothing useful

Anyways, anyone have a better idea? Please share more insight.

Appreciated.