Is it possible to use Pi's output signal as Arduino's input signal? I have a relay attached to an Arduino Uno which I want to control using my raspberry pi 3b. And yes I know it can be done by attaching the relay directly to Pi but I cannot do so. Don't ask me why. That is why I want to know whether it is possible to use Pi's output signal as input for the Arduino or not. I do not have spare parts so I don't want to experiment. If anyone knows about this, please tell me.
This way is fine: the Pi is a 3.3V based board, so connecting one of the GPIO pin from the pi to an Arduino will not damage it. Furthermore, the 3.3V are sufficient to be considered as HIGH even on a 5v Arduino like the UNO.

The other way around: connecting the output of a 5V Arduino to the input of a pi, will very likely damage the raspberry Pi! 

So beware of your Pinmode before doing such a connection!