A feature built into iOS is constantly tracking your movements and storing a detailed history of places you visit on a regular basis on your iPhone.
Also: How I learned to stop worrying and love USB Type-C
The feature, called Significant Locations, is described by Apple as follows:
Allows your iPhone to learn places significant to you in order to provide useful location-related information in Maps, Calendar, Photos, and more. Significant Locations are encrypted and cannot be read by Apple.
For some, this is going to be no big deal. For others, this will be seen as overreach on Apple's part, and an invasion of privacy.
Also: 10 Apple products you shouldn't buy (March 2019 edition)
So how do you find what information this feature has collected avbout you? Well, good luck finding it on your own because Apple has done an amazing job of burying it deep within the bowels of iOS.
This feature is located at Settings > Privacy > Location Services > System Services > Significant Locations. Note that you have to scroll to the bottom of the list in Location Services to find System Services, and Significant Locations is also at the bottom of the list.
In order to gain access to this data you will need to authenticate yourself using the iPhone's passcode, or using Touch ID or Face ID, so it's not like this information is available to anyone who has access to your iPhone.
Once you're in the Significant Locations screen, you will be greeted with a list of places you have visited, which is then further broken down into more detailed locations, along with dates you were at that place. Tapping on any of the entries brings up a map showing the precise location, along with the times that you were at that location.
To delete the data already collected you have two options:
You can turn off this feature completely by toggling the switch in the Significant Locations page in Settings. This prevents new data being collected, but it's important to note that it does not delete data already collected and stored.