The most common variant is for (effectively) every adult citizen to receive a fixed "tax refund" amount.
This is paid for by a combination of: replacing existing welfare with UBI, retrenching the bureaucrats that used to implement those complex welfare programs, and increasing the percentage of income tax paid.
There's some threshold where you might pay extra $20K on your income, but you get a flat $20K back as UBI, so you don't notice any change. This is typically somewhere in the middle class. Everyone poorer than this gets a boost to their income and/or have their existing welfare (and associated requirements!) replaced by an unconditional payment.
Everyone richer pays more tax, but not a huge amount more, since UBI mostly replaces existing welfare. It isn't an entirely new type of welfare on top of existing welfare -- this is the strawman that the right-wing likes to use in debates.
