I guess it may depend on the resource. If it's just a file, then consider having a "Resource Course" where the Authenticated User" role has the "View courses without participation" permission enabled. Then, you can directly link any File resource in this course into other courses. We do this with our School Calendar and Class schedules -- we update the file resource in this one "Resource Course" and the since this link is posted in many other classes, there is no need to run around and update the files there.
But, I don't know of a way to do this with activities where users submit, such as quizzes and assignments. You could consider creating a Meta Course (Moodle Doc here) or using the SubModule plugin (Moodle Doc here) to kind of pull separate course "units" to look like they are a part of one course, but both methods have ups and downs that may or may not be applicable to your situation.
But, I don't know of a way to do this with activities where users submit, such as quizzes and assignments. You could consider creating a Meta Course (Moodle Doc here) or using the SubModule plugin (Moodle Doc here) to kind of pull separate course "units" to look like they are a part of one course, but both methods have ups and downs that may or may not be applicable to your situation.