Well for starters there are already over a dozen different softwares in the fediverse. I can't even say exactly how many there are because some people have made their own blogs connected to the fediverse.
So that's the true power of ActivityPub, that it can be integrated into any software. Wordpress has a plugin, Nextcloud is working on one, Gitea is working on some type of federation.
Since I only ever used Mastodon (and GnuSocial before that) I can only speculate how exactly they work but let's assume you post a blog post on Wordpress, that blog post is then posted using ActivityPub to other nodes where your followers are. It might be posted to an AP relay that just relays the posts it gets to all other nodes connected to it. Increasing the discoverability of your blog to people who don't follow you.
So that means all AP nodes connected to that one relay just received your wordpress blog post in their inbox and will show that to their local users. (you have to differ between local and remote users in the fediverse).
And then the remote users of other nodes can comment on your wordpress post, and those comments are in a thread so you as the owner of the wordpress post will get those replies back to your instance, which is your wordpress blog.
So what I just described is you posting a blog post from your wordpress, but receiving comments on your blog post from other softwares, other nodes, readable in the comfort of your own wordpress blog.