If you open the Apps from inside the tunnel midway then neither Apple Maps nor Google Maps will start. It also depends on the configuration and openness of tunnels.
Maps need the internet through Cellular to start the navigation. Once started it does not require the internet it will work on GPS. (All Maps)
Maps rely on GPS (Global Positioning System) technology to determine your location and provide you with directions. GPS works by receiving signals from a network of satellites orbiting the Earth. However, these signals can be obstructed or weakened by physical barriers, such as buildings, trees, and tunnels.
When you're inside a tunnel, your GPS receiver may not be able to receive signals from the satellites because the tunnel roof obstructs the signals. As a result, your GPS device may not be able to accurately determine your location and provide you with directions.
Additionally, tunnels can also interfere with cellular network signals that Maps may rely on to determine your location. This can further contribute to inaccuracies in mapping data when you're inside a tunnel.
To overcome this issue, some tunnels may have their own dedicated GPS systems, which work by transmitting signals to GPS receivers inside vehicles. However, this technology is not yet widespread, so it's still common to experience issues with Maps when traveling through tunnels.