In V2 you could hit the /api/users/default path to get infomation such as the user\'s username. This is documented here: https://developers.google.com/youtube/2
In v3, user info is basically channel info.
By doing a channels->list "https://developers.google.com/youtube/v3/docs/channels/list", you will get the information under 'snippet.channelId' "https://developers.google.com/youtube/v3/docs/channels#snippet.channelId"