Here is an example of how this can look like https://framatube.org/videos/watch/da2b08d4-a242-4170-b32a-4ec8cbdca701.
This video shows PeerTube, a decentralized and federated video hosting platform and Mastodon and how they can interact with each other. In addition, I linked to the PeerTube instance on framatube but the video is actually hosted on peertube.cpy.re so we have another layer of federation here.
ActivityPub support in Ghost would basically be RSS/Atom on steroids, offering full interactions with posts. This post on Plume shows how a ActivityPub supporting blog post currently looks like in Mastodon and Pleroma, another microblogging alternative which uses ActivityPub as its internal data structure: https://baptiste.gelez.xyz/~/KaniiniTestBlog/current-status-of-plume-and-pleroma-federation/. So the service you’re using also can become a feed aggregator in which you can follow other blogs and post comments.
Come and join the Fediverse, it’s awesome