> money doesn't come out of thin air.
It can, though. Money is created all the time by banks only having to keep a fraction of their deposits on reserve. You deposit $100, they lend out $80, then you have $100 (in the bank) and someone else has $80 in cash (and an obligation to repay). $180 exists in a real form when at the beginning only $100 existed.
For the US Treasury, the rules are even stranger. Like a bank, they can write checks for whatever they want, as long as they do so in a manner consistent with the law (and laws can be changed). Unlike a bank, there can never be a "run" on the Treasury (no Treasury check can ever bounce for insufficient funds) because they can literally just print more money.
It is entirely possible to just deficit spend your way to a UBI. Many economists would predict this to create inflationary pressure - and indeed it might - but predictable inflation, even at moderate amounts, is beneficial to those who those who carry debts. This can create incentives for investment (you don't want to sit on your money if it's losing value) which may result in actual wealth creation.
Inflationary monetary policy effectively acts as a tax on those who hold currency, yes, but it's not a zero sum game. Plus people who receive these checks will spend it in the economy or deposit it with banks to create more of it. Economics is not well enough understood for there to be a consensus on what this would do in aggregate, and anyone who tells you otherwise is probably lying.