Manage access with UI
There are two ways to manage direct access to your WordPress files with AAM UI. Both are equally good, and you may choose either or go with a hybrid. It all depends on your website setup and access strategy requirements.
Manage access to media items
If you added files to the website through the Media Library and you see them on the list of media items, then you can choose to manage access to them with the help of Posts & Terms service.
In the example above, you can see that I added two files through the Media Library interface. In this case, you can manage access to each file independently. Select Edit media item, and if you have Render Access Manager Metabox option enabled (by default it is enabled), you should be able to the Access Manager metabox where you can set proper access controls.
Another way to set access controls to any media items is to go to the AAM page and select the Posts & Terms tab. Drill down to manage the "Media" post type and find your desired item. Then set your access controls accordingly.
Both ways are identical and it is just a matter of personal preference.
Manage access to media URIs
When files are not part of the media library or live in a completely different directory (outside of the /wp-content/uploads), then you may consider protecting them with the URL Access service.
FYI!
If the files that you protect live in the directory that is outside of the /wp-content/uploads, then make sure that your server redirect rules are adjusted accordingly.
For example, if you have a file that is located in the /wp-content/uploads/restricted/internal-compensations.pdf, you can create a new rule and deny access to that URI.
To do that, go to the AAM page and select the URI Access tab. Create a new rule and provide either the relative path or the absolute path to the file (e. g. /wp-content/uploads/restricted/internal-compensations.pdf or https://example.com/wp-content/uploads/restricted/internal-compensations.pdf). Then select the "Deny Access" option.
Conclusion
Managing access to the WordPress media files with Posts & Terms or URL Access service is equally good and, typically, depends on how your files are organized.
The general rule of thumb is that if your files are part of the Media Library, then go with the Posts & Terms service. Otherwise, use the URI Access service.