Does everything have to be connected for the phone to boot?

Probably a stupid question, but, does the 8 Plus need absolutely everything connected to the board for it to boot up? I know some earlier models don't. Otherwise, is there any other parts I could replace other than screen and battery, to try get it to turn on?

Also, with the home buttons, what is it that pairs them with the motherboards? I have the original button, with no flex cables. So would I be able to get a repair shop to put new cables onto this button, or is that not possible?

If that fails, then I'm just going to send it to a shop that does micro soldering and can have a good look at it.