Seconding Hello Chinese as a fantastic app. But I just want to share the apparently unusual-around-here experience that Duolingo was one of my best tools for learning and retaining Chinese vocab. I suspect it’s because the whole experience is fine-tuned to be vivid across so many sensory dimensions; when I remember a word I learned in Duolingo I can often also remember the exact place I was standing when I learned it. That does not happen for me with Anki or Hello Chinese. It could depend on brain type or something but I don’t think their success is purely a fraud.