The idea is cool with all the apps interconnecting, but where do I go to actually see this in practice. Does the cross communication only work on platforms like Mbin/piefed, or am I not seeing something.
I keep hearing thanks to activity pub posts here can be seen and replied to from elsewhere, but I haven’t seen it.
Evangelists say this but that’s not actually what happens in practice. It might one day. ActivityPub is not a protocol, it’s the concept of a protocol. Each platform uses AP slightly differently so there are subtle and big incompatibilities everywhere.
There are two main islands of interoperability. There’s Mastodon and friends (Pixelfed, Misskey, Akomma, etc) then there’s Lemmy and friends (Mbin, PieFed).
Mbin and Friendica do the best at federating with everything but they have shortcomings in other ways.
Just try stuff and explore, until you find a set of compromises you can live with. Take your time. I have a Mastodon account and a PieFed account.