When you plug a Voronoi into a Principled bsdf’s roughness and base colour, it makes some chunks rough and others smooth. I am wondering if it’s possible to somehow use an emission shader in similar effect, making only some chunks glow, without manually making the chunks in edit mode and assigning a material to each one. Also is thre a way to turn this voronoi into the cubes geometry, meaning that each chunk is a face, again without manually using the knife tool.
Yes you can use the voronoi texture to drive emission.
With regards to breaking an object - you need some sort of cell fracture. Take a look here:
Is there a way to fracture it randomly without using the grease pencil?
For simple thigs there is cell fracture which is an addon that comes with Blender.
For more complicated thigns you will want to use the fracture modifer build. It is in Blender 2.79 but you can just use it and then export the fractured stuff and import it into 2.9 either as obj or if you need it animated via mesh cache or alembic.