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This is baffling to me. When you use fast lens and you focus on a subject, you'd think that everything that's not in DOF range would be blurred but this is not the case if the object is a mirror or mirror-like reflecting the focused subject.
Case in point:
The smartphone itself is clearly not focused, it's blurry but the reflection on the phone shows the subject that's just as sharp. How is that possible?
In a plane mirror the image is at the same distance from the observer as the object. See the image below. The mirror is AB. The object is at C, the observer at E, and the image of the object is at F. The equality of angles of incidence and reflection gives us that the length of EDF is the same as the length of EDC, so if you focus on one you focus on the other. The phone is at D, so is at a different distance from the observer. The important point is that the image is not at the phone, it is somewhere else in space.
To be in focus, objects must be a specific distance from the image sensor. That distance is whatever the lens has been focussed at, and includes the Depth of Field range. Everything in the scene that is the same distance away from the sensor, will also be in perfect focus.
In this case the reflected image on the phone just happens to the right distance to be in focus. It is the mirror reflection on the screen that is in focus, not the phone or the screen itself.
See also Why is the focus distance for a mirror image further than the surface of the mirror itself?
