Not a cross post. It just creates a post that links to the first post.
If you made a third post that also linked to the first post using the same url as the second post, that would be detected and get linked up as a cross-post to the second post.
Actual cross-posts work in both directions. Lemmy detects them as the same thing, both posts link to the other.
Not in the post body, the post body quote and "cross-posted from" text is completely meaningless and just gets added by most clients, adding it doesn't make something a cross-post, and removing it doesn't break the actual cross-post feature, which shows up as a little "cross-posted to: list of communities" thing on the post page.
Thunder has it, Photon does it, and the default webUI does it.
Yeah - I think anything the UI is doing, it's getting the info from the API, so the poster would've had to use the 'cross-post' feature. There are some apps (e.g. Voyager) that try to wrangle cross-posts by title or URL, but title-matching can give false postives, and URL-matching usually assumes that one link hasn't picked up some cruft, and it can't do much for uploaded images if the poster didn't cross-post (because it'll be 2 different files with different URLs)
Oh, right. Sorry. I've gone back and checked the post I was thinking of when I made that comment, and - yeah - it turns out I was misremembering / didn't properly investigate the first time.