For more public information, you will want to access either https://api.picarto.tv/v1/channel/name/$CHANNELNAME or https://api.picarto.tv/v1/channel/id/$CHANNELID
To see everyone who is online all at once, you will want https://api.picarto.tv/v1/online
For private data, you will need to run a full OAuth2 flow. Further details are on the full API documentation.
For more details on endpoint output, as well as other endpoints, see https://docs.picarto.tv/api/