Yeah I heard a criticism of e.g. Duolingo the other day; even after two years on it daily, you'll know plenty of words and I suppose simple sentences, but you can't actually speak or comprehend the language, not unlike what the OP mentions.

I haven't learned a new language since high school, but I think to learn one you need immersion. Individual words for sure, but the focus should quickly go to reading whole sentences / paragraphs / books, listening to native speakers in their natural environment, and (probably the most difficult one to do on an app) speaking / conversation practice.

I only know English well because of daily exposure through media and twenty years of shitposting on the internet.

> I had good success with https://www.languagetransfer.org/

Yeah, I tend to think for the specific case of languages that share a common root, language transfer is unbeatable value (especially since it's entirely free).