@omi-kun I didn't get any video, I just at looked at the image you posted. And there is no subtitle in it, as well there isn't the button to enable subtitles.
Anyway, as you already found out, there is no transcription of the subtitles in that video. Aren't those subtitles automatically generated?
Try asking in a YouTube or video related forum if there is a way for you to download the script. Or try finding an extension.
I tried them but the extension of user string not working on unacademy u can try too.
I am not able to create an account there, so can't try. But, you can try some other user agent extensions. Or, you could use opera://flags/#reduce-user-agent and use one of the extensions for opera.com to force identifying as Opera. Then, that should make the addons site work.
@omi-kun There is option available in setting, go to setting> then click in advanced in Left side bar > then click browser, where you can add new shortcuts.