Backgrounds are for player characters. NPCs get whatever stats and abilities the DM thinks is appropriate, which for NPCs specifically labelled as "untrained" might be no proficiencies at all.Regarding trained vs untrained hirelings: who is an untrained hireling? Do NPCs not get background tool proficiencies? Shouldn't every adult be trained in some tool or useful skill? Or is that it, untrained is level 0, hasn't even gotten their background yet?