Besides data from smartphone apps, where else does the cell data come from?

I know that one can download a smartphone app and contribute data to OpenCellID. This is what is mentioned on the website: “The most effective way to contribute to OpenCellID is to use one of the smartphone applications listed here”. So while this is the “most effective way” to contribute, how else is data contributed?

Specifically, I’m wondering about remote and rural areas where cells are found. Did someone really go to these areas and record information with the cellphone apps? Or does UnwiredLabs attain some records from MNCs themselves, governments, NGOs, etc? I’m wondering about rural places in southern Africa as well as other locations. Thanks.