Nextcloud documents shared via controlled links that are displayed embedded in a file resource shows the complete site embedded

Create a nextcloud oauth2 service and a nextcloud repository.  https://demo.nextcloud.com/  is useful for this. Create a site wide instance of the nextcloud repository and only enable the "External" files. Add a file to the course and choose it from the next cloud repository (a txt file is a good example). Edit the file and open the file picker again. Select the nextcloud repository and then press the small "Logout" button in the file picker. Go back to the course and view the file. You should see a button like "Login to your Nextcloud account" in an iframe in the course page. The iframe should not contain all the surrounding navigation like breadcrumbs, course header etc - it should only contain the button.