A one line combination of YouTube-dl and ffmpeg can do this btw (without downloading the whole video)
ffmpeg -ss ${seconds} -i "${first_url_from_yt-dlp}" -t 1 -r 4/1 -q:v 2 -vf scale="0:-1" ${target}
-r 4/1: 4 frames per second
Again this app is for mobile devices which lack yt-dlp+ffmpeg
Keep in mind you need to make sure to use input seeking, though (-ss before -i).
Also I'm not sure why they need `-vf scale="0:-1"` part.
If yes, could you please share for me here, it'd be really helpful, thanks
But with a little bit extra work you could make that one liner into a script that will extract a range of frames surrounding the chosen timestamp.