Locations for external libraries

The default install directory could be misleadingly listed as /usr/lib/modules, but this actually points to ~/snap/steam/common/.local/share/steam.

You can verify the actual install directory by opening the Steam client and navigating to Settings > Storage.

Note the full path listed above the storage space indicator:

You can add external storage libraries as you normally would through Steam Settings > Storage, but the snap is restricted to libraries located in ~, /media, /run/media, or /mnt.

This same restriction applies to adding non-Steam games.