Retweets Branch, Everything Else Accumulates
A like is terminal. It increments a counter and nothing follows. A reply extends the post but keeps the audience the same. A retweet does neither — it copies your post into a new follower graph and hands the people in it the same button.
That recursion is the whole mechanism. Someone with 800 followers reposts you; three of them repost it onward; one of those has 40,000 followers. Nothing about your account changed, but your post is now somewhere it could not have reached by any amount of engagement inside your own audience.
It also explains why repost counts look small relative to likes and still matter more. Reposting is public and permanent — the post sits on your timeline attached to your name. People are considerably more selective about that than about tapping a heart.