Why my back camera and flashlight not wrking?

I'm going to disagree with @kingofspades regarding the two components you're having trouble with. The rear cameras are on two flex cables, one for each camera, while the flash is on a completely separate flex cable along with a microphone and the buttons on both sides of the phone.

I haven't encountered this myself, but I've heard from others on here that it's not uncommon for the flash to be disabled when a camera is malfunctioning. The camera replacement is fairly straight-forward so my suggestion would be to try replacing the rear cameras first then reevaluate.

iPhone 7 Plus Rear-Facing Cameras Replacement - iFixit Repair Guide

I suspect that once the camera is working again the flash will recover by itself, but if not you may have to consider replacing the flash/volume/microphone flex cable. Unfortunately iFixit doesn't have a guide for that, but the Spanish site Nadie Me Llama Gallina has the one you need.

Manuales / Apple iPhone 7 Plus / Flex buttons, microphone and flash

You can get the parts you need right here on iFixit if you want to support their repair efforts.

iPhone 7 Plus Dual Rear Camera

iPhone 7 Plus Audio Control Cable and Brackets

If you are budget conscious you can easily find less expensive parts on other sites like Amazon, eBay and AliExpress.

Good luck; let us know how it goes!