Q1: I learned that YN685 has a build-in receiver. It seems to be pretty convenient, so why do most people buy YN568???
I'm not sure most people do buy the YN568 EX. A lot of YN568 EX flashes were sold before the YN685 was introduced for Nikon cameras. Possible reasons to prefer the YN568 EX over the YN685:
- The YN-568 EX is about $10 cheaper than the YN685
- The YN-568 EX is slightly smaller and lighter than the YN685
- If one only plans to use the flash mounted on a hot shoe the wireless receiver is superfluous, more expensive, and heavier
Q2: If I wanna have TTL for YN568, does that mean I absolutely have to buy 622 flash trigger because RF603 does not support it?
Yes. You need either a YN-622N-TX transmitter or one YN-622N II transceiver attached to the camera's hot shoe to control the YN685 in i-TTL. You also need a YN622 transmitter to use HSS (Auto FP), 2nd curtain sync, or FEC/FEB.
Q3: If I purchase a YN685, do I have to get a 622TX?
No. You could just use the flash mounted on your camera's hot shoe.
But if you want to use the i-TTL and HSS radio capabilities of the YN685 then you need either a YN-622N-TX or YN-622N II.
If you only want to control the power and zoom settings of the YN685 wirelessly as a manual only flash, you could use a YN-560N-TX transmitter or a YN560 IV or YN660 flash mounted on the camera.
If you only want to fire the YN685 wirelessly as a manual only flash, you could use an RF-603/RF-603 II/RF-605 transmitter.
Q4: If I get a YN685, what happens if I use it with RF603?
You can set the radio in the YN685 to receive YN-560N-TX/RF-603/RF-603 II/RF-605 radio signals.
You'll only be able to fire the flash wirelessly with the RF603/RF603 II/RF605. You won't have i-TTL, HSS (Auto FP), 2nd curtain sync, FEC/FEB, or remote control of the flash's power or zoom settings. The flash power and zoom head settings will need to be set using manual power control on the control panel of the flash itself.
With a YN560-TX transmitter or YN560 IV/YN660 flash on your camera's hot shoe the YN685 will also receive radio signals. You will be able to wirelessly control the flash's power and zoom settings, but only with the YN685 set to manual flash. You still won't have i-TTL, HSS, 2nd curtain sync, or FEC/FEB capability.
It's all covered quite well at Flash Havoc.
Q5: For taking selfies with YN685, can I just hold a YN622TX and take pictures or do I have to leave YN622TX on the camera's hot shoe and get a camera remote??
No. The YN-622N-TX needs to be connected to the camera for the flash to sync with your shutter.
Yes, you need a remote. Some flash triggers include the capability to trigger the camera via the camera's wired remote port when another trigger in the same system is used as a remote. Most can't act as both a receiver to trigger the camera AND as a transmitter to trigger the flash at the same time. However, if you can use a slow enough shutter speed you can sometimes get the camera and flash in sync by triggering both with the remote in your hand. It depends on the capabilities of the trigger system you're using and some fairly involved setup that is beyond the scope of this answer.
Q6: If I get a YN685, and a YN622 TX. I put my YN622TX on my camera, my YN685 flash on a stand. Attach 2 RF603 on 2 strobe lights, can I fire them all at the same time when I take a picture? And does my YN685 has to be on manual mode?
No. The YN685 and YN-622N-TX are capable of receiving signals from the RF-603/RF-603 II/RF-605/YN-560-TX/YN-560 IV/YN-660, but the RF-603/RF-603 II/RF-605/YN-560-TX/YN-560 IV/YN-660 are not capable of receiving radio signals from the YN-622N system. They are two different radio protocols. The YN-622 system components made since the end of 2014 can receive either protocol, but only transmit in the YN-622 protocol. The units in the RF-603/RF-603 II/RF-605/YN-560-TX/YN-560 IV/YN-660 system can only transmit and receive their own protocol.
If you want to mix i-TTL and manually controlled studio flashes your best bet is to go with the Godox system.
For more about that, please see How to connect YN560tx to canon speedlight 430 EX II?