From your location to the contacts in your phone book, apps can gather a broad array of data from your mobile device.
The best and most efficient way to cut down on the number of companies that may have access to your personal information is to delete any apps and their respective accounts you don't use. Purge your app library and get rid of programs you haven't opened in a while, especially apps you have may have downloaded for a specific event like a festival or a conference.
You can also manage which apps have access to certain aspects of your phone through the settings menu on iOS and Android.
On your iPhone, you can get started by launching "Settings" and scrolling all the way down to view the apps installed on your phone. Tapping an app will display what types of data and parts of your phone that particular app has permission to use. From there, you'll be able to enable or revoke access. For example, tapping Google Maps will list the permissions that it requests, such as your location, Bluetooth sharing, microphone, and cellular data among others.
The process is similar for Android devices, although Google presents it differently. Open the "Settings" menu, choose "Apps & notifications" and press the "Advanced" option. Then choose "App permissions" to see a list of all the different permissions apps can request access to. This includes data and components such as your contacts, calendar, call logs, and location, among others. Tapping each category will allow you to see which apps have access to that information and revoke access if desired.