One thing all IoT devices have in common is connectivity. In most cases, this means that the device can connect to the internet; in some cases, it might communicate with a local smart hub via another technology such as Bluetooth, which is connected to the internet itself, and sometimes devices from a local network, for example, via radio (see the ZigBee and Thread sections). In each case, there is a communication channel, so devices can either send out data, receive commands, or both.
Having a device that is connected to the internet or another device is much use. It needs to be connected to input and output components. With sensors as input components alone, there are a gazillion possibilities. If you haven't done so already, you should have a look at the sensor category for Sparkfun, one of the best-known shops (https://www.sparkfun...