...
Activity and course completion interact through course completion's "Activity" criteria. This criteria checks a user's activity completion state in a course module.
Activity completion
This is set up via an individual course modules settings, and once set work on user's future activity within the module. There are some default criteria, and some modules have additional activity criteria defined. Once user's have completed the activity, the settings become locked and cannot be modified unless all existing completion data is deleted.
User data is stored in the table "course_modules_completion".
Recording time of activity completion
When activities are completed that are part of course completion criteria, Moodle and Totara both update a timecompleted field in the course_completion_criteria_compl table.
...
When completion criteria is updated with the chosen option of keeping existing completion records, the relevant records in the course_modules_completion table remain and no changes are made to them.
Course completion
Course completion is decided by enabling completion criteria for a course. Criteria are checked by a cron job, and if required criteria are completed a user is marked as "complete" in a course.
User data is stored in the tables "course_completion_crit_compl" for individual criteria, and "course_completions" for the aggregate results for a course.
Criteria
Course completion criteria:
...