When discussing Fediverse, you sometimes hear the idea that you shouldn't need different accounts for different platforms.
The "why can't I comment under Youtube video with my Twitter account".
In Fediverse it already works to some extent. You can comment under PeerTube video with your Mastodon account, and you can participate in Lemmy thread with your Mastodon account.
What is this functionality called? Does it have specific name?
It's probably not "single sign-on" and not "federation" from what I understand.
And to what extent is it implemented? Is there some way to participate in Lemmy thread with your PeerTube account?
Is there any progress being made on implementing the initial idea of using any Fediverse account with any other Fediverse platform?
What is the name for the problem of implementing or mapping the platform-specific functionality? As in, neither Mastodon nor PeerTube have the interface for upvoting and downvoting posts.
Wouldn't it be easiest to implement Fediverse-wide SSO (as in, log into Lemmy interface with your Mastodon account) instead of figuring out some funky mapping and patform interactions?

Online dating could probably be implemented with federated technolgies.

Are there any projects that are already working on this?
There are some open source dating apps, but they are mostly Tinder clones. It would probably make more sense to reimplement something like old OkCupid, where it's mechanics and systems worked more for the benefit of the userbase.