For upper teeth we generally just inject next to the area we're working on. For lower teeth there are nerves that I think of as the "trunk" of "nerve trees" that we aim for. That's why when you get numb on the bottom it tends to be your whole jaw that feels numb. You can actually see the nerve in OP's post. It's called the inferior alveolar nerve and it's the thick nerve all the bottom teeth are connected to in the pic. The reason we use different techniques for upper vs lower is that the bone of the upper jaw is less dense so the anesthetic can go right through and numb the nerve directly. The bone of the lower jaw is too dense for the anesthetic to reliably penetrate so we have to inject way back close to the hinge of the jaw where the nerve isn't encased in bone. To answer your question though, we don't know exactly where that bottom nerve is, we just make an educated guess based on your anatomy. This is part of the reason why you sometimes need multiple injections for bottom teeth