if you have closed captions for a different language in your youtube video but you're speaking english, there is no way to just fall back to auto-generated subtitles. you have to use google translate built-in on the subtitles that exist and it is noticeably worse.
my brain is slightly nonstandard and i like having subtitles on even when i can hear and understand the voice lines, because then i really get what's being said. otherwise it just kinda falls out of my mind relatively quickly