You know, I have no idea.
I think it depends on the linked-to servers.
If the server thinks the Lemmy thumbnail/summary generator is a bad bot, it will not return a thumbnail. If it’s actually a decent web server, I think the Lenny instance will generate & distribute a thumbnail automatically and attach a summary.
I presume this is handled by the posts hosting instance.

I’ve noticed some sites that get linked to don’t have a thumbnail and some sort of “prove you aren’t a bit” subtext/caption.
And others work fine