If you are using the Set Designer “Sets and Locations” they come in .LIB4D format. This video covers how to install them and use them in Cinema 4D.
The .LIB4D format is one of my favorite features of Cinema 4D because it makes it easy to create a local database of models and materials. And then easy to share and move those libraries to other systems.
If you have any questions please head over to the forum or leave them directly on the YouTube video.