The ActivityPub spec actually has a Server2Client part, which is where a regular “ActivityPub server” like lemmy or mastodon or anything like that could talk to an “activitypub client”, which would be capable of interacting with any activitypub server and display it correctly.

AFAI, nobody bothered to implement it anywhere, but it is possible. It is also very much not like the internet as a whole, as activitypub represents user interactions in general, not anything ever.