For example, if I'm feeling stressed/anxious, I'll scroll/browse/distract myself to avoid the negative feelings. I'm not seeking them like doom scrolling says.
You're right that in general it's about getting those random dopamine hits when something nice appears in the news feed.
However, after some time, you got a lot of the nice stuff and no exciting stuff appears anymore. At that point, you're still scrolling, hoping for a dopamine hit. It does not come because you are satiated, desensitized and the algorithm no longer has good stuff to offer you.
I get it here on Hacker News. After coming too often and scrolling too much, I already clicked on all the good links. All that is left is either not interesting, or stuff I've looked at before. I still scroll, doomed to find nothing. And yet I scroll.