Why my iPhone won't boot?

Your phone was either damaged from falling on the hard porcelain or from getting infiltrated by water, possibly a bit of both. Even though the iPhone 7 is water resistant, it is still possible some water got inside. At this point the phone must be opened and cleaned properly asap.

• Open your phone and remove the logic board (follow this guide)
• Inspect the logic board, especially around the connectors and look for corrosion.
• Inspect both sides of the board. Unfortunately, most of the board is covered in shields. That's usually where the damage is occurring.
• Put your board in a container with >90% isopropyl alcohol and let it sit for a while.
• Use a soft brush, like a toothbrush and lightly brush away any corrosion you see.
• Rinse in alcohol and repeat.
• Let it air dry for a day.
• Re-assemble and hope for the best.

If the phone still doesn't work, then a micro-soldering repair shop can properly troubleshoot your phone.