Very beautiful question ! Maya is not an illusion, it is what appears to us and what we think of as ultimate reality, i.e. we normally think people are different from me, there is good and bad, here is this world, here are my problems, I am so and so, etc. It is neither existence nor non-existence. It is not existence as it is intrinsically dependent on Brahman to exist and keeps changing. It's not nonexistence as it appears to us as a dual world as a part of our experiences.
The Maya of the Vedanta, in its last developed form, is neither
Idealism nor Realism, nor is it a theory. It is a simple statement of
facts — what we are and what we see around us. It admits that this
world is a mixture of good and evil, happiness and misery, and that to
increase the one, one must of necessity increase the other. Like moths
hurling themselves against the flame, we are hurling ourselves again
and again into sense-pleasures, hoping to find satisfaction there.
here. We return again and again with freshened energy; thus we go on,
till crippled and cheated we die. And this is Maya. With every breath,
with every pulsation of the heart with every one of our movements, we
think we are free, and the very same moment we are shown that we are
not. Bound slaves, nature's bond-slaves, in body, in mind, in all our
thoughts, in all our feelings. And this is Maya. (Swami Vivekananda,
Jnana Yoga Chapters 3-5)
How do we define falsity in Advaita Vedanta? That which “borrows” its
reality from something else is false. It does not have an existence of
its own. Similarly, all that appears in the universe, seems to
exist. Their existence is borrowed from Brahman, the infinite
existence, which is your own Self. False is not non-existent. So, that
which you cannot say that it truly is, and that which you cannot say
that it is not, this mixture, which neither exists, nor can be said
to be non-existent, this is the false or mithya. (Dissolve Into
Infinity, Swami Sarvapriyananda, Pages 20-21)
Because when there is duality, as it were, then one smells
something, one sees something, one hears something, one speaks
something, one thinks something, one knows something. (But) when to
the knower of Brahman everything has become the Self, then what should
one smell and through what, what should one see and through what, what
should one hear and through what, what should one speak and through
what, what should one think and through what, what should one know and
through what?(Brihadarankaya Upanishad 2.4.14)
Acharya Sankara writes, " Because when, i.e. in the presence of the
particular or individual aspect of the Self due to the limiting
adjuncts of the body and organs conjured up by ignorance, there is
duality, as it were, in Brahman, which really is one without a second,
i.e. there appears to be something different from the Self. Śruti
says, ‘Modifications are but names, a mere effort of speech’ (Ch. VI.
i. 4-6 and iv. 1-4), also ‘One only without a second’ (Ch. VI. ii. 1),
and ‘All this is but the Self’ (Ch. VII. XXV. 2)."
