I bought my most recent AP at the hotel concierge (changed a ticket to an AP). I didn't need to do anything special to activate it, and I think the clock starts ticking on 1st day you enter park, not the day you buy it. I'm not 100% on other buying options.
If you put the AP into
MDE then it will be on your resort magicband.
You still need to carry the AP card to receive any discounts, plus CM CAN ask to see the AP or ID when entering the park. It is worthwhile, IMO, to carry it. You also need to
REMEMBER to show it to CM's, at every restaurant, gift shop, and WDW hotel to receive discounts.
If you log into MDE, you can ALSO request your AP magicband, so in theory you qualify to have have two magicbands. Another option - likely the better option - is to request the APmagciband, because the AP magciband comes with a special little slider band. (and perhaps decline the resort magicband - if you have enough time before your trip to receive the APmagciband).
The special little slider band says AP on it. Mind, the slider is
just for fun, not proof. For discounts, you have to remember to show the actual AP.