> remove a brain from a body and it is no longer concious either

What is no longer conscious, the brain? Or the body? Or some other entity?

If consciousness is weakly emergent, how do we know it emerges from the solely from the brain and not, say, brain + body? Or brain + body + or environment. Or from the universe itself?

You can draw a line somewhere. Peole argue against conciousness by constructing (deliberately or not) an example where the line is drawn in a silly place.