For example in the screenshot (twitch clip), if you want to share on reddit you click the reddit button, but what button are devs supposed to put to share in the fediverse? Do they pick an instance of their choice?! Is this a weakness of a federated platform or what?
What you’ll likely see is the button links to a centralized service that sends you to the Lemmy instance you are logged into
So the button would never link directly to any Lemmy instance but some central server that sends you to your own instance.
Who would run that server? Probably the same guys who develop these button integrations.
Or it could just require you to set up your login on your instance in the settings and then work as a regular share button