One thing I don't understand, the clients (and servers) have different features ? So how are they compatible with each other ? I understand the activitypub allows interoperatibility, but if piefed for crosspost agglomeration but lemmy doesn't, how is that mismatch handled ?

Are they extensions that are just not supported like on XMPP ?

I think I'd want to run each of the 3 webui locally, to figure out which I actually like, but does that mean I should runt he 3 backends as well so that all the features work ?

Is there one backend that basically does it all and I should only run that one ? I do have plenty over ram with many HP G8 DL380 and 256gb ram each so .. it's mostly a matter of "crossing the desert" and configuring each one, which I suspect will be pretty similar but each with their own little gotcha (speaking as a self-hosting veteran, there's always something)