To get some of the additional details such as if a user is subscribed to the channel, or a Twitch Prime user, you have to request the tags capability, as documented here: https://dev.twitch.tv/docs/irc/guide/#twitch-irc-capabilities you will there have tags on the irc messages that you can parse and check for if they have the Twitch Prime badge, or a sub badge, etc…
As for the subscribers API endpoint you’re attempting to access, if you read the docs you’ll see that you need to use an OAuth token as authentication with the channel_subscriptions scope. This token has to be generated by the broadcaster of the channel you’re attempting to access subscriptions of going through your apps auth process.