There does not appear to be a way to get the channel associated with a user. So if you have (only) a UserID, you have to lookup the username for that id, then lookup the channel id for the channel with the same name as the username, then get that channel’s details by channelID.
/users doesn’t give the channelID, just the UserID. And nowhere is it stated that UserID == ChannelID (In fact, it’s implied they’re not).