It would be a good idea to try with a keyboard plugged in but the flashing folder with a ? On it usually means that it cannot find an operating system. Try to restore your system if you still have the discs. Update (03/02/2018): You could be looking at a failed hard drive and that’s why your iMac cannot find the operating system. It would be a good idea to try to recover your data.
You got a lot of poor answers here. And some just unnecessarily blunt. But judging by your questions wording I think you probably meant the home button flex cable? You refer to Phone size... The plain answer is no. The iPhone 6 and iPhone 6 Plus home button flex cables are of different length and so the cable for the 6 cannot be used on a 6 plus. Other than having extra cable folded inside your 6 I don’t see any reason that the cable for the 6 plus cannot be used. This part itself does not affect the functionality of the touch ID. As for the home button itself. What they say is true. Although the button and it’s assembly can be interchanged between these devices, once replaced you will lose all functionality of Touch ID. I don’t know about this bricking problem after updating. I’ve never run into it and I do this replacement all the time.