Siddhartha says "maybe things all are semblance, maybe not; very well, then I too am only semblance." During which experiences did he come upon this?
He says this in the last chapter to Govinda. Does he reflect these thoughts in any of his previous experiences in the book?
I think that Siddhartha's understanding of semblance is a point at which he has arrived throughout his narrative's experiences. I don't think that there is "one" particular experience that indicates this to him. Rather, I think that it is something that comes to him over the course of what he experiences and the knowledge that he absorbs as a result of such experiences. This understanding of semblance is something that arises out of the idea of the meaningless of life and the search for understanding and meaning within it. Throughout Siddhartha's experiences, reality in both personal and external conceptions had been endured to this search for meaning. It took him the full understanding of these entities to grasp how he, as a human being, is part of something larger and that what exists in front of him is part of this configuration, as well. This notion of semblance is what enables Siddhartha to cease looking for meaning outside of the world and rather see himself as part of it, within which meaning is evident if one understands its true nature. This was not a conclusion arrived at by one single event. In trying to isolate one event, I think that one moves away from the idea of semblance that Siddhartha accepts at the end.