On 12.x build 57dc8c9e, I uploaded a file to Resources. In Resources, I copied both types of URL's from the Resource file's Edit Details/Folder Details area: Web Address URL and Short URL. In Lessons I created links using both URL's (Add Content-Add Content Links-"Or add a URL")
After import into another course (Site Info > Import from Site > I would like to replace my data), in Lessons, both the full web address URL link and short URL link worked correctly for both instructors and students (students were not in the original course site). Students could access the file.
On 19.x build ff87b46c and 20.x build aef80c92, I followed the same steps I did for 12.x above. But after import, in Lessons the students couldn't access the Resources files using the file's full web address URL nor using the file's short URL. Instead, student's get a HTTP Status 403 - Forbidden error after clicking on either link.
The web addresses (full URL and short URL) in the imported course look like they match the web addresses in the original course.
Note for 19x./20.x: After import, students could access Resources files linked to using Add Content/Add Content Links and "Or select existing files from Resources".