Hey
I just finished an introduction article about FPC (Flex PCB). It's no more than an overview of what flex technology is, why it is more costly, some designing rule of thumbs and how to order from a manufacture. Dave's #1262 episode was a masterpiece, as usual, and useed as a resource in it.
Have a nice reading!
https://atadiat.com/en/e-an-overview-of-flex-and-rigid-flex-pcb/