We've two locations  for adding cohorts to courses:
1. Participants list - enrol users - select cohort
From my understanding this adds the users in a cohort in this moment. Its not dynamic
2. Course enrolment methods_ cohort
If I use this the setting its dynmaic and new members of a cohort will be enroled when they are added to the cohort.

Its confusing that the diffeences are notexplained.
I suggest to add tooltips here:

Idea
For 1:  This enroles just now existing  members of the selected cohort to the cohort.
For 2: This enroles the members of the selected cohort to the course. Cohort changes will add/delete user enrolements.