The anode needs to have continuity to work, the Teflon or PFTE fills the voids to create the seal but the threads cut through and make contact. If you would like to verify this put some tape on screw it in and then remove, you will see small areas have cut through the tape and this is all that is needed.
Magnesium > Aluminum
Aluminum anodes have a few issues:
Aluminum anodes tend to swell and break up as they are consumed, aggravating sediment and raising blockage potential
Aluminum in our bodies is...not the greatest thing in the world
If an aluminum anode passivates, it may be very difficult to tell what is going on
Aluminum anodes can be difficult to ...
I go with zinc because aluminum swells more and when aluminum breaks down it can plug flow restrictor and ariators. you will need the model number of the water heater to get the correct length. I find solid rods last longer I have segmented break at the joints several times.
In 1 home I installed an electronic annode it was very expensive almost 200$ but ...
Metal water pipe isn't grounded to minimize corrosion, it's grounded to protect people in case something goes horribly wrong and the pipe gets energized. With PEX, the pipe is an insulator, so grounding is both effectively impossible and not beneficial.
If you were to ground any water pipe or fixtures, you would tie them to the house ground, not the water ...