YouTube has had a toggle to open a transcript of a compatible video for some time now. However, it has only been available on via desktop browsers so far. Now, however, it has reportedly been spotted on the app's Android version. The feature seems to have been heavily modified for its new format.
https://www.notebookcheck.net/YouTube-ports-its-transcription-feature-to-mobile-starting-with-Android.607993.0.html