I'm other words, how does the querying application know which resource it needs? How do you know that a pixelfed instance will get the pixelfed resource versus the mastodon resource?
I can't speak for those two services in particular, but I know that Matrix will check https://domain.com/.well-known/matrix/server to see what (sub)domain is responsible for domain.com. I suspect other services also use .well-known too.
I've thought about doing this. It seems simple enough, though it would need a front end to be user friendly. I could build the rest api pretty quickly.
I think what they mean is a service to provide the webfinger response in a customizable manner. I know for me right now, I'm just using wildcards in nginx to handle it for the ActivityPub path, but it's not really ideal because it won't work for other activitypub services as far as I can tell.
there is technically nothing stopping you from listing multiple different accounts under a same webfinger query (hell, lemmy does that because it supports groups and people with the same names) but most fedi software will not handle it properly and will pick an arbitrary one of the two.