You need a ticket AND a reservation. They aren’t the same thing, but you’ll book your reservation when you buy your ticket.
Tickets will either be
Disneyland only, DCA only, or Parkhopper. If you get a park hopper ticket, you must choose a starting park (either DL or DCA) and you can only be in that park until 1pm. After 1pm you can go back and forth to both parks as you like.
Can you change your reservation? Yes, but technically no. In order to change your reservation you need to cancel your existing reservation and book a new reservation. This isn’t that complicated, but there is a SLIGHT danger that when you cancel your existing reservation and go to rebook, nothing will be available. If that happens, you’ll need to stalk the site for availability. If for some reason you are unable to make a reservation, you are out of luck.
Genie+? There is no more fast pass. Now that system is called genie+ and it costs $20. There are no more physical kiosks to get fast pass tickets, instead everything is done through the app on your phone. In that regard it’s like the old Max pass system. A major difference is that you can only use the FastPass lines (now called lightning lanes) once per ride. Yes, you should get it if you are focused on rides.
There’s also three rides that are not part of genie+: Radiator Springs Racers, Webslingers, and Rise of the Resistance. If you want to use the lightning lanes for those rides, you can purchase those passes individually. It will definitely get you on those attractions faster if you find that priority access to be worth the cost.
That’s the overview of how it works. There are many other threads if you want strategies or details on how best to use these to plan your park day.