Here is a VERY brief answer: You can use Geosearch to search for a frequency, frequency range or FRN associated with a fixed site/location (i.e. filtered by State, County etc, or Coordinates).
The "location" could be a repeater, or a base station, or a bunch of mobiles used within a certain radius of (for example) a school or factory. ULS data has a code for site type, but I don't believe it's accessible from the ULS web API, which really sucks (you can access at lot of data fields that aren't on the Web API if you download raw ULS data files and import them into a local SQL database).
Honestly, you can easily spend hours and hours and hours deciphering the intricacies of ULS. It is NOT very user friendly and web queries with even a few seemingly simple filters can take forever. I'm guessing ULS is based on a VERY ancient database platform and convoluted data format/structure that was inherently difficult to interface with a web API (I'd be curious to know specifics)...