In a situation like yours where there's no sign of life whatsoever, the place to start is the power source. If the power can't get into the phone, it can't power the phone and can't charge the battery so there won't be any functions working at all.
Have you tried a wireless charger? Wireless charging uses a completely different input mechanism to get power to the motherboard and then the battery, so it may still work even if the lightning port is totally fubar.
In theory you could also check that by swapping out the battery for a replacement that will normally have a certain amount of charge and see if it powers on. Assuming you don't have a spare battery lying around, I would try replacing the charging port as the next step after trying wireless charging. That's what connects the lightning cable to the motherboard so if that's failed you won't have any power getting to it.
Unfortunately, iFixit doesn't have a guide to that particular repair, but there's a Spanish site called Nadie Me Llama Gallina that has just what you need. The site is in Spanish but if you use it with the Chrome browser's translation, it works quite well.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/apple-5/apple-iphone-12-pro-1296/flex-de-carga-11575|Manuales / Apple iPhone 12 Pro / Load flex - Nadie Me Llama Gallina|new_window=true]
There's a copy of Apple's official repair procedures on the Device page, but that assumes you're using Apple's equipment and won't necessarily give you the best guidance for doing it yourself.
[document|27199|Apple's iPhone 12 Pro Repair Guide - iFixit]
The part is available for sale here on iFixit in whichever color you need to match your phone if you want to support their repair efforts.
[product|IF442-010|iPhone 12/12 Pro Lightning Connector Assembly|new_window=true]
You can also find it on most of the major sites that carry phone parts such as Amazon, eBay and AliExpress. The color selection is strictly cosmetic; in practice you can use any color and the only effect will be what you see on the inside of the charging port.
So I'd suggest changing out the lightning port flex cable and see where you are from there.