iPhone The Videos app includes an Alternate Track button 
, which you can tap to choose subtitles and captions offered by the video you’re watching. Standard subtitles and captions are usually listed, but if you prefer special accessible captions, such as subtitles for the deaf and hard of hearing (SDH), you can set iPhone to list them instead when they’re available.
Prefer accessible subtitles and closed captions for the hard of hearing in the list of available subtitles and captions. Turn on Settings > General > Accessibility > Subtitles & Captioning > Closed Captions + SDH. This also turns on subtitles and captions in the Videos app.
Choose from available subtitles and captions. Tap
while watching a video in Videos.
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Customize your subtitles and captions. Go to Settings > General > Accessibility > Subtitles & Captioning > Style, where you can choose an existing caption style or create a new style based on your choice of:
Font, size, and color
Background color and opacity
Text opacity, edge style, and highlight
Not all videos include closed captions.