Did anything happen to the device in the days and weeks leading up to this issue, such as a drop, a repair attempt or contact with water?
Assuming this was a spontaneous failure, it is probably a failed component on the logic board. The iPhone 7 is prone to several issues after repeated minor drops and flexing over the course of its life. It could also be a dead or dying battery or modular component.
If you have access to a multimeter, you could measure the battery first. If it reads ~3.8V - 4.2V, then it is good.
I would start by testing a bare-bones setup. Start by disconnecting the battery first (always!). Then disconnect the digitizer, LCD, front camera, rear camera and home button extension flex. Now re-connect the battery; you should only have the battery and Lightning Dock flex connected. Connect a known-good, preferably Apple-original Lightning cable to an iTunes enabled computer.
Does iTunes recognize your phone? If so, then you should re-connect one flex at a time until the phone stops working. I would start with the Home button flex first, then add the rear camera, LCD & Digitizer then from camera flex.
If the bare-bones setup does not get recognized by iTunes, then you most likely have a logic board issue. You could send it (or find a local) to a repair shop that specializes in micro-soldering repair. Sometimes the fix can be straightforward (i.e. a blown filter) and relatively economical.
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