Installing GrapheneOS via the Webinstaller on a Google Pixel
GrapheneOS is considered a very secure operating system. It is the OS of choice for people whose data must be protected at all costs. Like journalists, whistleblowers, …. It comes without any third-party applications or services, only free and open source software. It is not recommended to use it as an everyday phone. Third party applications can indeed retrieve data without your knowledge despite your vigilance.