I can answer a narrow part of your inquiry: Why do you need to sink the rod into the dirt, and merely touching it isn't good enough? Dry dirt is a very poor co

I can answer a narrow part of your inquiry: Why do you need to sink the rod into the dirt, and merely touching it isn't good enough? Dry dirt is a very poor conductor. Wet dirt is pretty good, thanks to the dissolved salts, but still not great compared to copper. Most dirt is somewhere in-between, so the amount by which you must sink your ground rod to be safe partially depends on your local climate. Lets say you have a 3/16" grounding rod sunk three feet into the dirt. The cross section through the rod at the top of the rod is ~0.025 inch^2. But the total contact area on the local dirt is ~42 inch^2. That's an increase in area by a factor of over 1500x. The local dirt doesn't need to be particularly conductive to work as a reasonable conductor when you have that much contact area to work with. The total cross sectional area of the dirt-conductor around your ground rod also gets larger quickly, proportional to r^2. That term gets large rapidly with distance, such that almost all of the resistance between the planet as a whole and your ground rod is concentrated at its interface to the dirt.