Correct, there is no learning material. FCC is an ongoing process made with volunteers. That section didn’t get filled out for that version of FCC but presumably will be for the next version that is in beta (though things will be organized differently, including the certs.)
When I did the DV cert, I just did my own research. For SASS, I just looked at some web pages - it was pretty easy to pick up. For React and D3 (both much harder) I spent about a week each just looking at youtube instructional videos (including some stuff on the FCC channel) and got enough to get started (though I still stumbled a little).