Thank you Collin and Mary.
I know about Role and permission and the rest. Not everithing but yes the "basis" . The same for the rest mentioned above.
David exactly said what I would like to know : "... if that same ability (... changing default activities settings.- on the top of completion settings..) exists for all settings for all activities at the course level. For example, if a particular teacher preferred online text submissions in the Assignment activity and rarely used file submissions, is she able to set the default settings for the Assignment so that each new instance was automatically set for online text - even though the site-wide default might be for file uploads? "