In iOS Settings > Privacy you will find you can control apps access to Contacts, Calendars, reminders, Photos, Bluetooth, Local Network, Microphone, Speech Recognition, Camera, Health, Research Sensor & Usage, Homekit, Media & Apple Music, Files & Folders, Motion & Fitness. As Ira L. indicated the app must first request access to the device/feature/function which happens the first time you open the app. After that, you can turn that specific access on or off at will in Settings > Privacy.
