The basic premise is that everyone is taxed some percentage of their income and then they are paid back some flat amount. For those (probably) in the middle class, this tax/payment will be exactly the same. You pay X per year and gain X per year in benefits. For those who make less than that, they will end up earning more from the payment than they pay in to the system. Those who earn more money than that will pay more into the system than they receive back in benefits.

>Do only certain people qualify for basic income?

The good part about it is that everyone qualifies for the exact same payment, this reduces the cost of administering the program significantly. No need to fill out forms, check if you are covered, prove you make under a certain amount, or talk to 20 different agencies, the only thing that has to be confirmed is that you are a legal resident.