This is a good article. It provides insights on what exactly each carrier collects, a more recent run-down of how long each United States telecom retains certain types of data for, and images of the tool the FBI makes available to law enforcement agencies across the country to analyze cell phone tower data.
https://www.vice.com/en/article/m7vqkv/how-fbi-gets-phone-data-att-tmobile-verizon