You will not get any money until you get back on A-day and look at your bank account and see the deposit of your paycheck ($225 if you pay the initial $2000, $175 if you don't) for the months of July and August.
Any other funds that are mentioned to you are, in essence, monopoly money that you hear about and never see. The cadet account is a strange work of banking magic and even when you do get access to topoff, how it all works is still pretty mysterious.
Unless, of course, you go down to penfed and have them explain it to you. I've done that, it really isn't all that interesting but has a lot to do with the fact that you don't actually see what you get paid.
Cadets make about $990 a month, but only see a percentage of it. The rest goes to pay for things that you'll use.
Let me be clear, the $2000 is for your laptop that you will be issued after Beast. The laptops we are issued are considered personal laptops that we have purchased. Which is why your pay gets docked if you don't supply the $2000.
This is explained in detail when you receive the laptop. If you don't pay the $2000, then the expense for the laptop comes from your pay for (I think it's just the first two years) some time until the expense is paid. If you don't have the money for it, don't worry about it. It won't be that bad.
Bottom line is, all you need to think about concerning money is:
1. West Point is "free"
2. You get a paycheck once a month that is a percentage of what you make in total
3. The $2,000 goes towards computer stuff.
Anything anyone else tells you about advances and money is an attempt at explaining the banking behind how everything is funded, which isn't all that relevant to everyday Cadet operations.
You asked why can't they take the initial deposit from the 7200 advance? The answer is that if you don't provide the 2000, they do take from that advance. Part of the "advance" (it's really more like a loan) is the pay you actually see. It's not like you get this amazing advance of money and also get paid. It all comes from the same pot. So when you don't pay the $2000 initially, the same pot has less money in it. Your cadet pay also comes from this pot.