how come Lemmy shows more upvoted posts further down the thread instead of at the top?

what decides what goes up and what down?

The sort setting

Are you sorting by Hot or Active?

https://join-lemmy.org/docs/users/03-votes-and-ranking.html

Here is an explanation for each sorting type. It sounds like you're looking for "Top". I personally prefer Scaled so that I get some content from the small communities I'm subscribed to

I like chronological feeds

I believe "Hot" sorting filters by a mix of recency, score and possibly also activity. If you want to sort by score alone, you can use the dropdown to change the sorting to use "top" instead. There may also be an option in your settings to make this the default if you want to.

Brand new posts get a boost in the ranking

Just a note that comment sort is currently broken on Interstellar app, but they are working on fixing it. You can change the default in settings and that will work, but not the menu in posts.

I'm on the boost for lemmy app and noticed this happening a few months ago. Seems to be the sort option is broken so shows new by default and needs to be adjusted each community. Once I fix it in each community, it stays that way.