Nothing is stored on the MagicBand except for a unique id number.
Everything is linked to your MyDisneyExperience (MDX) profile. This includes room reservations, park admission (MYW Ticket or AP), FP+ selections, etc. The MagicBand is just a key used to read the data stored on your MDE profile.
Why do people keep calling it a "Key". Yes I know that is proper database term for it. But Key implies locks and security.
It's more like a the address/s on a business card. It tells people where to look for and how to get in touch with you. In this case your magic band is a business card for your MDE account. And each magic band associated with you and your MDE account points to all the same information.
As the PP said, nothing is STORED on your Magic band except for some very large, basically random, unique numbers. Tonight if I have time I'll dump out one of my Magic Bands, I have software/hardware that reads NFCs at home (I have it here at work too, but I don't have a Magic Band here right now.)
We are annual passholders and took our first trip over Memorial Day weekend with Magic Bands. My husband grumbled about wearing something on his wrist (neither one of us wears a watch) and I thought he would hate them.
Nope - he loved his. He grumbled when we didn't have charging stuff since I hadn't set up a credit card with the band since I thought he would hate the bands. He really liked just having one thing for his admission and his FastPasses. He said he liked the freedom of not having to dig for his wallet (where he kept everything before) all the time. Hence his desire to add the credit card to things. He loved just waving the band and moving on.
I just put mine into my bag and waved it wherever and then returned it to the bag.
We entered our AP info on MDE and then it went right onto the Magic Bands for park entry. Like the PP mentioned, we don't wear watches and I thought I would hate it. I loved it!! Super easy and way more convenient then getting my AP out and passing them out to everyone at gate entry. I only had to get my actual AP out when I was using it for a merchandise discount