inter symbol interference vs digital modulation
ISI is an (typically undersired) effect of the channel – that can be a cable, an optical fiber, free space or the air you use to transport your wave.

The different kinds of constellations are a method of putting data in a waveform.

So, they are connected: The modulations define which symbols your system uses. ISI is what happens to the symbols when you rapidly send symbols one after the other.

Hence, your receiver needs to deal with ISI, and it needs to understand how to get the data back from the symbols it received.