The Get Broadcaster's Subscribers and Get Broadcaster Subscriptions endpoints do the exact same thing (they shouldn’t even be 2 separate entries on the reference, they are the same endpoint, just one uses an extra querystring param).
All you can do in Helix is get a list of subscriptions to a channel, and optionally use a querystring param to filter the results to see if specific users are subscribed to a channel. It is not possible with the currently documented endpoints to see who a user is subscribed to.