Which flex cable did I cut?!

Seems pretty obvious that you've cut the lightning port flex cable. If you look at the part being sold here on iFixit, the second (back side) picture shows exactly the area you're indicating.

iPhone 12 Pro Max Lightning Connector Assembly

Looks like iFixit hasn't quite gotten around to making a guide for that repair, but luckily there's a Spanish site called Nadie Me Llama Gallina that has just what you need to do that repair.

Manuales / Apple iPhone 12 Pro Max / Load Flex

Of course, since iFixit depends heavily on users helping to make guides, you can always add to the knowledge by taking lots of pictures during your repair and using them to create your own lightning port flex cable assembly replacement guide! It's fun, easy and will be greatly appreciated by everyone who comes after you.