iPhone 7 not starting, no display, not detected by itunes

A bit of history on the device would be helpful. Was it dropped, repaired or came into contact with water?

Here is some general info that may help you out. If you have access to a multimeter, you could measure the battery first. If it reads ~3.8V - 4.2V, then it is good. Then 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 or come back here and post your findings so we can give you more direction.

UPDATE

If you suspect a logic board issue, check PP_BATT_VCC, PP_VCC_MAIN and PP5V0_USB. I would start by checking to see if those rails are shorted to ground. If one of these rails is shorted to ground, then you will need to identify what is causing the short. It could be a bad decoupling capacitor, conductive debris or defective IC that is directly supplied by those rails.

Then you move onto the PMIC and check the voltage rails it generates:

• The PMIC generates ~15 voltage rails. They are all important (for obvious reasons) but the ones to check first are as follows: PP_CPU, PP_GPU, PP1V8_SDRAM, PP1V2_SDRAM, PP_VAR_SOC, PP0V95_FIXED_SOC, PP3V0_Tristar, PP3V0_NAND, PP1V8_ALWAYS and PP1V0.
• Also check VDD_BOOST
• The PMIC also generates, what I would consider secondary, yet still important voltage rails for the following sub-systems: PP3V0_MESA, PP1V8_VA_L19_L67, PP3V0_PROX_ALS, PP3V0_PROX_IRLED, PP3V0_IMU, PP3V3_USB and PP3V3_ACC