That's exactly what I do. From videos I use losslesscut [1] to isolate the sentences I want to practice on and then turn those into anki flashcards with some custom tools I wrote.
Speech to text (via google cloud speech) and dictionary lookups (via a custom gui). Only downloaded files, luckily the sources I use make their content available online for free (and I wrote some python to automatically download new episodes).
If you want to download from YT i recommend NewPipe, better than every dodgy browser extension (i shall install another program on my machine? GTFO) i tried. You can just get the audio too. Android only though .