Yes, each instance has its own subreddits (known as "magazines" or "communities") which will often overlap.
However, you can subscribe to a subreddit from any instance (unless the instance mods banned it). So you can subscribe to @
technology@lemmy.ml, @
technology@beehaw.org, and tech on kbin. When you search for "tech" subreddits you will find all three, and if you subscribe to all of them then you can interact with them in the same way regardless of their host.
That sounded weird to me at first, too. But then I realized it's not
that different from reddit, where I also occasionally subscribed to multiple overlapping subreddits (e.g. /r/politics and /r/politicaldiscussion). And when I scroll through the headlines, I usually don't pay attention to which subreddit it came from.
BTW, kbin and lemmy are no different in this regard. Kbin has simply recreated a lot of subreddits from Reddit, but often users have gravitated to the lemmy instances instead. For example, right now a lot more users are on @
technology@lemmy.ml and @
technology@beehaw.org than on tech on kbin.