Can you learn ASL from an app, or does that only work with a Deaf teacher? Specific Language

Apps are fine for the vocabulary layer and genuinely bad at everything else, and in ASL the everything else is most of the language. Facial grammar, spatial referencing, classifiers — an app quizzing you on isolated signs is teaching you a word list, not a language. Use one for the word list, and get real instruction for the grammar, ideally from a Deaf instructor. Community colleges often run evening classes and your employer's 'something modest' very likely covers one.

From the other side of your counter: on day one the thing that matters isn't signs, it's behaviour. Face the person, don't talk while looking at your screen, don't grab to get attention — wave or tap a shoulder — and always ask how they prefer to communicate rather than deciding for them. That's an afternoon of learning and it changes every interaction before you know a hundred signs.

Regional variation is bigger than people expect. The sign an app teaches you may not be the one used locally, and unlearning it later is annoying. Local instruction gets you the local variant the first time, which is a real practical argument for the class over the app.

Video, never photos. Any resource showing you a still image of a sign is close to useless, because movement is part of the sign. If either of your apps is picture-based, drop it today.

One practical note for a clinic: whatever you learn personally, your employer very likely still has obligations about proper interpretation for anything clinical. Don't let 'the receptionist is learning ASL' quietly become the accommodation. Worth raising with whoever handles that where you work, early rather than late.