There are several levels of no separation.
On the level of the small self, whatever I see as external to the small self is also there internal to the small self. What I see in you, I recognize from myself. It may be there only as a potential, or unfolded to various degrees, and it may take different external form, but the quality is the same. This opens up for spontaneous empathy, and a deep sense of us all being in the same boat. We are all in it together. There is no separation.
And when I awaken to my own nature as space & awareness, free from any fixed identity and distinct from the world of phenomena, I see the world of phenomena as a fluid seamless whole. It is one fluid system. There are distinctions but no separation.
When I awaken to myself as space & awareness, I also see that everything – all phenomena – unfold within this space & awareness. It is that from which all phenomena emerges and that in which it unfolds. There is no difference here, and no separation.