Generally, it’s based on appropriateness to the C/, effort, and usefulness.
There are exceptions, though there aren’t any on C/s that I actually use. But there might be eventually, there were on reddit.
If something doesn’t fit the C/, that’s a down vote if I notice it.
If something is horribly low effort, even if it’s in the right place, that’ll be a down vote.
Upvotes, it tends to be because something was appropriate to the C/, and/or someone put some work in. But, even a low effort post/comment can get an up vote if it’s personally useful.
I used to up vote anything and everything that was on topic, but lemmy has gotten busy enough that I tend to only vote at all if I interact with the post in some way. So, like a title that indicates the post isn’t something that will interest me, I just scroll past because there’s just so much stuff now. But, I scroll All, and sort by new by default, so I end up scrolling past stuff that isn’t in my subscription list. If I only scrolled through subscribed stuff, I’d probably end up voting the same amount, but voting on everything I saw, if that makes sense.