Whenever I approach Illusionary Magicks, under the house of Magic in general, its usually in two formats:
Mental and Physical.
Mentally-manipulative Illusion Magicks fall under the category of spells related to the control of ones senses, particularly their eyesight and their ability to perceive things around them. For instance, a use of mentally-manipulative Illusion Magicks would be switching the scents of two types of tea, or taking the picture of someone else's back, and swapping it with another.
However, in my personal works, this has certain limitations. For instance, to swap the scents of two teas, you must first have the knowledge of both scents yourself to a high degree. As for the back-swapping, it would work like this: If Person-1 is intent on following Person-2, and Person-3 wants them to believe they are following Person-4, then Person-3 would stand behind Person-4 and 2, take an observation of their backs, and then cast their Magic spell to imprint their vision of Person-4's back on Person-2, and vice-versa.
Then, there is Physically-manipulative Illusion Magicks.
This one is trickier, given it actually falls far under the aforementioned control of senses, particularly sight and touch. Through an in-depth knowledge of the way one thing feels and appears, you can replicate it onto any surface imaginable. This also messes with one's mind. For instance, if Person-1 perceives an illusionary wall made out of metal to be metal upon contact, they are to assume that it is, indeed, made of metal.
This would deter them from trying to hit it without a proper reason, as it would likely hurt to hit the 'metal'.
Now, it gets interesting for physical illusions.
Causing things like phantom pains require one to have felt that pain themselves. Such as, if you've been suffering from a blocked nose, you may cast an illusion to fool the target's mind that they themselves have a blocked nose, regardless of their condition. This can be used in a multitude of ways, and if the caster is prepared enough - it can kill.
By imprinting the feeling of their own death onto their target, it will instantaneously kill the target - but it does, obviously, require the caster to first die, and then have a system set to instantaneously cast the illusion spell.
I only have one case of illusion magic in my story so far, and that could be considered softcore gaslighting. It's effective invisibility by telling the minds of whoever witnesses it that 'This isn't important. Don't think anything of it.' But as others have said, it depends on the author that writes it. Like most other elements of fantasy, magic varies pretty drastically between works, and that includes illusion magic.
That actually reminds me of a creature from Dungeons and Dragons - The False Hydra.
What it does is, simply, sing.
Its song makes all those that hear it automatically perceive that the Hydra is not there in the first place, and even if the Hydra attacks and eats someone, everyone will entirely forget about the person in question, as if they had never existed in the first place, with everyone creating an excuse for gaps in logic present.