…Is this  “add-on” plugin a modifier to the ActivityPub plugin? Would I add the code to the plug-in itself, or make a whole different plug-in and install it alongside?

The Block Editor, Custom Meta, and ActivityPub by Jan Jan (jan.boddez.net)

I’ve been using WordPress’ “new” block editor for quite a while now, have moved my site over to a block theme, and so on, but I also almost always have a couple “old-style” meta boxes open.
And because of how WordPress works, this combination of Gutenberg and meta boxes results in posts …