For more public information, you have to access either:
https://api.picarto.tv/api/v1/channel/name/$CHANNELNAME or https://api.picarto.tv/api/v1/channel/id/$CHANNELID
To view everyone who is online all at once, check: https://api.picarto.tv/api/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://api.picarto.tv/