If you’re familiar with SSH keys, it’s basically the same thing but written for everything to use rather than just written for SSH.
Passkeys can also be managed by TPMs/device hardware to provide private keys that (assuming things are properly designed) can’t be stolen and are bound to the device itself. This isn’t a requirement however, which is why things like 1password, Chrome, etc can sync passkeys.
They’re basically syncing the one-or-more private keys that back your logins.
So, Mastodon is working on a communities feature. I think having Mastodon and Lemmy communities have interop (along with great interop between the “same community” on two different instances) would be the superior option.
As an example, if someone posted or tooted the same link to several communities I’m following, I should be able to see all of the comments aggregated/tab between the community posts and responses. In other words, it should aggregate a view of all the different discussions about that link for all the communities I’m subscribed to and/or that my instance knows about.