mastodev, ionch
@f0x in almost everywhere on mastodon - and the rationale used by the W3C in not mandating webfinger - your "@name" is a link to your personal account page, and any button that does any action to that Person generates as a part of its Post or URL query params, the address of your Person object. So you can "follow your nose" to the object in question, as they say.
mastodev, ionch
@f0x ? yes and no.
No matter what, webfinger requests for me have to be directed at `https://cybre.space/.well-known/webfinger?resource=acct:falkreon@cybre.space`
They could then produce URIs for 'self' and 'salmon' that live on different servers, but in practice they never do.
For the purposes of ActivityPub my real user id is the id of my ActivityPub Person object, or `https://cybre.space/users/falkreon` (the url given as rel='self' in webfinger).