If you have a link to the 21 library then just try changing the "21" to "20" in the link and see what happens.
But this will probably not work if the link refers to a build number as well as you need to know that as well (like in the help > check for updates).
I do this also to change from "AUS" to "INT" or "USA" if I want to download a different version and it works most times for the Archicad installation files.
But generally I don't think you will find a direct link to the libraries because they are part of the Archicad installation.
From Archicad you can go to the HELP menu and check for updates.
Maybe your local distributor has a link to your local version library?
One of the forum moderators. Versions 6.5 to 26 Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10 Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11