I'm not sure exactly what you mean by "reservation systems," but I'll give it a go here.
If you're booking your trip through Disney, you can book a package, which includes room and park tickets. I'm not sure if you can even do this anymore, but years ago you could also book your flights as part of this package. There are certainly other vendors through which you can purchase everything as a package--flights, car rental, room (wherever it may be--onsite or off-site), and tickets.
If you book through Disney, there are different up-front payments required as well as different cancellation policies for a package or a room-only (no tickets or anything else, just the room) reservations.
If you are talking about the park reservation system now in place, that's something else entirely. You have to have tickets in hand in order to book park reservations and you can reserve only one park per day and you can only reserve that park if it's available to be booked. Lately nothing is selling out, but park reservations have sold out in the past.
Once you book your park reservation, if you decide you want to book a different park for that day instead, you have to cancel the park you booked and then book the new park. Yes, you could end up being shut out of both parks. Not a great system, but it's the one we're stuck with right now.
If you have a hopper or AP, you can hop under these circumstances: (1) You must tap into your reserved park. You cannot hop to another park if you haven't tapped into your reserved park. No exceptions. (2) You cannot hop until 2 p.m. No exceptions.
Regarding
Genie+: I'm guessing you purchased a package from Disney. This is the only way, I'm pretty sure, that you can buy Genie+ for your entire stay. FastPass does not exist anymore. It was "replaced" by Genie+. I have that in quotes since FastPass (and FP+) were free, but you have to pay for G+.
You cannot book G+ reservations in advance. At all. You can book your G+ reservations starting at 7 a.m. on the day you're making the reservations for. At 7 a.m. you can book one attraction only. Two hours after park opening time, you can book your 2nd attraction. There are dedicated threads delineating all the particulars about this stuff. No point in going into it all here.
Hope this helped you a little bit. Things have changed a lot in 23 years!