To say "everything is one thing" goes against many things we've been taught and firmly believe. Using our everyday perspective, we see a house over here, and a car over there. How can the house and car be the same thing? (Even saying something like "everything is one thing" is inaccurate because that framing places a boundary around the 'one thing'.)
"Different" is defined as: not the same as another or each other; unlike in nature, form, or quality.
"Separate" is defined as: forming or viewed as a unit apart or by itself.
Does "separate" imply "different"?
"Different" applies to the underlying nature of something, whereas "separate" applies to a perspective.
When it dawns on you that everything is part of a grand singular scheme, that perspective may seem to come out of the blue, or it may be something you've nurtured for a while (or may still be nurturing!). Either way, figuring out this perspective is a major paradigm shift.
Along the way, you’ll surely encounter a bunch of smaller and seemingly less consequential ideas leading to this conclusion-- each one chipping away your everyday sense of reality.