In my experience, when I cross-post using the built-in Lemmy cross-post function (the two little squares between the star and dots menu below the post header), it shows up like what is in this example post. In other words, it shows a relative link to the cross-post on both the original article and the cross-post. The cross-post link on this article where we are currently discussing this shows me "cross-posted from: [a hard-coded link to original post on solarpnk.net]", and the original post doesn't mention that it's been cross-posted elsewhere.
I'm not complaining, by the way, I'm just curious what causes this because I have seen both styles for a while now. I had previously assumed that people were manually cross-posting and adding a link to the original, but then I just realized that that's probably not the case because those types of cross-post look very consistent.