It takes a fair bit of work to produce quality subtitles. So in my experience most youtubers/videocasters rely on machine generated subtitles. The quality on these are sometimes great, often okay, but sometimes pretty bad. It doesn’t currently deal well with all accents, and can struggle on technical subjects that use unusual verbiage (like uncommon words, domain-specific terms, or pronounced acronyms). Still, even mediocre subtitles can help at times.