A magnetic drain plug is exactly what it's called: an oil drain plug that is magnetic. What it does is attract small metal particles that get into the oil as teh engine wears. When you pull the drain plug out to change the oil, the metal particles come out with it. Just clean the plug and reinstall. It's just an extra piece security in addition to the oil filter. I have one in my Nova. I also have a magnet in my Novas transmission pan and the rear axle housing cover.
The block and head are aluminum but not the entire engine. Plus, aluminum blocks have steel cylinder sleeves and rings. Aluminum would wear too quickly and does not seal as well.