There are transporter bots that admin configure to run in the background. These are what transport the likes and comments between instance servers. You are connected to one of these instance servers. LW has a large volume of interactions so some instances may limit how often or what is transported by the bots between servers. It is all done with activity hub protocol. Take a minute in a desktop browser to view the page source for Lemmy and you’ll better understand what is involved and how information is passed around. It looks like structured text. This is pretty easy to spot even if you are unfamiliar with code. You’ll likely understand why the message exists about DM not being private too, and maybe understand why blocking works poorly on Lemmy as that is pretty much implied.

If an instance defederates, what that actually means is that the admin is not allowing the automated background bots to carry activities between the two instances. Everything up to that point is already synchronized and does not go away.