In System Services, you'll find a full list of what built-in Apple services you have granted access to your location data, which may be more extensive than you knew. Some of these options need your location for daily use, such as Find My iPhone and location sharing. But at the bottom of this list, there's a tab called "Significant Locations," which requires your password (or fingerprint or Face ID) for access.
Under Significant Locations, you'll find a comprehensive history of all the locations your iPhone has been.
The list is sorted by city, and under each city, you can find even more specific info — the exact addresses of places you've been, how many times you've been to each place, the specific dates (and times!) you've been to each location, and how much time you spent at each place. It's a wild amount of data.
Before you turn off your Significant Locations feature altogether, don't forget to clear the history that's been kept on your phone. The "clear history" option is located all the way at the bottom of your history list.
Finally, you can turn off the Significant Locations list by toggling off the feature at the top of the page. Apple may still have access to a huge trove of your data, but at least you can keep it from compiling a detailed list of everywhere you've ever been!