On Desktop Enhancer for Youtube has this - https://addons.mozilla.org/en-US/firefox/addon/enhancer-for-youtube/ Though I thought Firefox had Picture-in-picture as a browser feature... I can't remember since I never use(d) it.
But clearly you're asking about mobile. No idea if this or any other extension is supported there.
For Japanese, the core 2k/6k deck I have is really good. Has examples sentences, audio (for both word and sentence), and pretty well formatted furigana. I believe it's the one linked here: https://djtguide.neocities.org/anki
And not quite Anki, but I've been trying JPDB and it seems pretty cool. Combines Kanji/Kanji parts learning with vocabulary, and includes pre-made decks with vocab from various media. But the cool part is the decks combine - so once you've learned a word as part of one deck, you don't review the same card in a different deck (at least I think that's the idea? Still trying it.)