Hi,
The Library Download is one of the 'deployment options'. It lets you self-host your DTM library files. With the Library Download hosting option, dynamic tag management sends a ping (an HTTP post) to your system to let your servers know there has been an update to the library files.
Your IT team can create a cron job, or other scripted process, so your servers can pick up the changed files. In this way, whenever a change is made in dynamic tag management, the files you are hosting on your servers can be updated automatically. Post this setup is complete, the URL is generated and it's used by DTM to make an HTTP POST call.
If you already have this setup for your DTM company, please contact your IT team who would have access to the library location within your environment and would be able to share the library files with you.