The lore is that Magearna was brought to life by the device (Soul Heart) in the center of its body storing up the life energy of various pokemon (the movie shows how this was done).
I haven't seen the movie, but I just read a plot synopsis. So, the Soul Heart drives Magearna, and when it is removed Magearna's body is lifeless. The Soul Heart still contains Magearna's consciousness when that happens, but it's in the form of energy. Magearna's consciousness has some say in where that energy goes and how it is used, but ultimately it must go into something designed to use the Soul Heart as a power source. It was also originally empowered by draining the life force from living things.
So it's an energy being created from the life forces of living creatures. It's basically a ghost. The Soul Heart just contains it and allows it to be harnessed to power things, such as the robot body that most people think of as Magearna. I still think it's possible that Marshadow is that ghost, released from the Soul Heart. They have very similar facial proportions (round head, red vertical oval eyes, tiny mouth about level with the bottom of the eyes, widow's-peak-esque markings), their names start with the same first two letters, and Marshadow comes right after Magearna in the Pokedex (assuming the order the dataminers got is Pokedex order).
If the purpose of the Soul Heart might not be to make Magearna more powerful, it might be to contain the power of Marshadow in a way that it can be harnessed for specific purposes. Breaking it free from its confinement might make it more powerful and is probably what it actually wants.