The general rule for illusions is on p.203 of the PHB:
Illusion spells deceive the senses or minds of others. They
cause people to see things that are not there, to miss things
that are there, to hear phantom noises, or to remember
things that never happened. Some illusions create phantom
images that any creature can see, but the most insidious
illusions plant an image directly in the mind of a creature.
So, what you see in the illusionary mirror is whatever the creator of the illusion wants you to see subject to the capability of the particular spell or other effect that creates the illusionary mirror. Some spells like Minor Illusion are static and limited in size so those limitations will impact on what can be shown, others like Phantasmal Image actually have the target "filling in the blanks" so that they autocorrect any errors in the illusion.
If the caster knows what a real mirror in that location and orientation would show then they can chose to show that. Irrespective of their knowledge they can show what they think is around the corner, or what they make up, or an image of the 347th layer of the Abyss, or a close up of a dental filling, or anything else you can imagine that you want to put in a mirror.