Does the flash YN 685 have a built in receiver? In some places I see that it is written that it does, but other places don't mention it.
Depends on what you mean by a receiver.
The YN-685 has a built-in YN-622 radio receiver in it and can be remotely controlled from a Yongnuo YN-622-TX transmitter or YN-622 transceiver with HSS/FP and iTTL. It can also be fired, and have its power, group, and zoom controlled from a YN-560-TX or YN-560IV/YN-660. However, this manual-triggering scheme does not allow for HSS/FP or iTTL as the 622 units do.
The YN-685 has no optical slave capability and cannot be used as a Canon near-infrared wireless eTTL slave, a dumb optical slave, or a Canon -RT radio slave.