Basically, given that MR itself exists on a spectrum, its use of the physical and digital worlds can vary. What makes it different, however, is that although it has the ability to transport users to the virtual world, it also takes into account the physical environment that the user is currently in and uses that to create a digital experience. A common example of this is if a user has a desk in the room, the MR tech would take this into account. If there was something hidden under that desk, the user would have to physically look under that desk in the real world to be able to access whatever is hidden in the digital world.
Given that MR operates on that spectrum, the extent to which it actually transports users to the digital world or augments their physical environment will differ for different systems. It is predicted that headsets that can cover the entire spectrum will be a reality in the near future.
On the most basic of levels, those are the fundamental differences between VR, AR, and MR. Virtual Reality transports its users to the virtual world without consideration for their physical world. Augmented Reality only takes in the physical world to augment it for the user. Mixed Reality combines the functions of both VR and AR. At the same time, all of these fall under the overarching term of Spatial Computing, given that they enhance how users interact and receive data from their computational devices and make them more intuitive and natural to humans.