Webmentions are implemented with my own Webmentions plugin. (The plan is to open source that very soon as well, but I need to do some cleanup before I am comfortable sharing it.)
I also implemented my own Mastodon and Bluesky integration to display likes and comments below the posts. This is pretty baked into the site right now, so I am not able to share that part as of today. (I should make this a separate plugin as well eventually.)
Florian