YN622c TTL question
" .. How will the camera know that the actual speedlight is not on the camera, but instead is off camera .. "
Within the limitations of ETTL II, it won't and to a large degree, it doesn't matter. Off cam Rx units are/can be set to Groups. Canon Wireless Light type handles Groups A & B differently to Group C. I havn't seen reports of whether Yn 'honour' those differences or not - my guess would be that they do.
The outgoing control, power setting & triggering are now all Radio, but the return part of the logic loop is still light and it's still measured by the camera, TTL.
Practice lots, experiment for you between Eval for flash & Average for flash and keep in mind that this tech is all reflective mode metering based. So the photographer still has to be thinking about the tone & reflectivity of the scene for every shot - and tweaking FEC accordingly. Not expecting to use FEC frequently is every bit as daft as using automatic ambient exposure and expecting never to use EC or ELock. Have fun!