It mentioned in REDCap feature and Help:
REDCap API - Have external applications connect to REDCap remotely in a programmatic or automated fashion.
als check this out: https://www.iths.org/blog/news/redcap-tip/redcap-api-101/
But I couldn't find any document or training material for API either. I am not the REDCap administrator and don't have the right to "touch" the source codes. I sent out e-mail to ask for those things from our University REDCap administrator, but not sure if they WILLING to give it to me or not.!
REDCAp_API1|690x470