Live Transcribe in the accessibility settings. AFAIK it's available on any fairly recent Android phone. I bought a Pixel tablet for no other reason but to run it -- nothing else I've tried comes close for local-only continuous transcribe-as-they-speak. (iOS has a similar feature also under accessibility; it's good but not at the same level. Of course I'd love to see an open-source solution.)
This was for English. One problem it took me a while to realize: when I switched it to transcribe a secondary language, it was not doing it on-device anymore. You can tell the difference by setting airplane mode.
There's a captioning button under the volume slider, and I think it's called "live captions" or something in settings. Just tap the button and it'll start.
