Buddhism learns that everything is in constant change and in constant interaction; that nothing is steady and separated from the other things. Does that mean that buddhism is nondualistic?
Yes it certainly is nondualistic, however Buddhism goes beyond simple nondualism, effectively assuming perfect loss of (attachment to) any form not just the illusion of "I". So you no longer hold position such as "everything is one" either. You don't even hold a position such as "everything is so" or "I should not hold on to any position". It's a complete and utter abandonment of any position.
Original Pali Buddhism does not teach about 'non-dualism'. Instead, it teaches about 'voidness' (selflessness; sunnata).
The world is empty...In what respect is it said that the world is empty? Insofar as it is empty of a self or of anything pertaining to a self: Thus it is said, Ananda, that the world is empty.
In a number of places, it states there are internal & external phenomena.
The six internal media should be known. The six external media should be known.
Owing to a dyad (pair), monks, consciousness comes into being. And how, monks, does consciousness come into being owing to a dyad?
"Owing to the eye and forms arises eye-consciousness. The eye is impermanent, changing, 'becoming-otherwise.' Forms are impermanent, changing, 'becoming-otherwise.' Thus this dyad is fleeting and transient; impermanent, changing and 'becoming-otherwise.' That cause, that condition, that gives rise to eye-consciousness — that also is impermanent, changing, becoming-otherwise.' And how, monks, could eye consciousness, having arisen dependent on an impermanent condition, become permanent? Now the coming-together, the falling together, the meeting-together, of these three things: this, monk, is called 'eye-contact.' Eye-contact, too, is impermanent, changing, 'becoming-otherwise.' And how, monks, could eye-contact, having arisen dependent on an impermanent condition, become permanent? Contacted, monks, one feels. Contacted, one intends. Contacted, one perceives. Thus these states also are fleeting and transient; impermanent, changing, 'becoming-otherwise.'
MN 1 lists the perception of 'oneness' ('ekatta') as not Nirvana & as one a many possible experiences.
He perceives earth as earth...He perceives water as water... fire as fire... wind as wind ... beings as beings... gods as gods...Pajapati as Pajapati...Brahma as Brahma... the luminous gods as luminous gods... the gods of refulgent glory as gods of refulgent glory... the gods of abundant fruit as the gods of abundant fruit... the Conqueror as the Conqueror ... the dimension of the infinitude of space as the dimension of the infinitude of space... the dimension of the infinitude of consciousness as the dimension of the infinitude of consciousness... the dimension of nothingness as the dimension of nothingness... the dimension of neither-perception-nor-non-perception as the dimension of neither-perception-nor-non-perception ... the seen as the seen... the heard as the heard... the sensed as the sensed... the cognized as the cognized ... singleness as singleness (ekatta)... multiplicity as multiplicity ... the All as the All ... Nirvana as Nirvana...
This essay may help: Dhamma and Non-duality.