Yes, they are automatically generated by Rapidmooc from the prompter or PowerPoint notes as .vtt (WebVTT) files and .srt. WebVTT is the official text track standard for the web (full details can be found on the web standards organisation website here: https://www.w3.org/TR/webvtt1/). And, SRT stands for SubRip Subtitle file, which is a caption file made up of time coding and plain text. Video software effectively extracts the sequencing, time code and text information to display subtitles on a video. They can both also be easily converted to other subtitles/closed caption formats as needed.
Please see an image of a video with subtitles:
There are two example subtitles files (.vtt and .srt) attached below.