Judging from personal experience? I’m going to have to say under the fingernail into the nail bed. If you’ve never experienced someone having to cut away part of a nail because of an infection, consider yourself blessed because holy shit that hurts.
Without seriously mutilating the nail bed in the process it should fully heal within a couple weeks, baring infection or something else, though damage to the nail itself can last for a couple months while the nail slowly pushes the damaged part off the end, like a passive aggressive cat claiming the dining room table.
I’ll be honest, I don’t even know what trigger warnings to put on this, since “tw ohgodwhywouldyoudothat!?” doesn’t seem to be a thing. So, preemptive apologies for the rest of you that didn’t want to think about this.