A lot of the "magic" of UBI ideas is the time value of money and a lot of the same fundamental concepts as other forms of passive income. One way to think of it is your Government making a massive investment in "short term loans": your UBI payment is a loan of $X on April 16th with an expectation to pay Y% of $X back on April 15th of the following year (as the taxes you owe on the income you made that year). If Y% is greater than or equal to 100% it is exactly a loan, with interest. If Y% is less than 100% it is a government subsidized loan. A lot of the debate inside UBI discussion is what is the most effective Y%, and a lot of the beliefs about UBI is that you can do it with a surprisingly small Y% for among the same reasons that the Federal Reserve can give 0% loans to most banks. If they can give such favorable terms to short term loans to banks, why can't they give that to average citizens? When you look at all the complex deductions in the existing tax code as existing loans with favorable percent and add them together, that also drops your Y% and potentially simplifies things. (Instead of deducting lots of individual line items, you shift Y% individually, the terms of your personal annual loan agreement. Or you can leave Y% consistent and adjust $X up to replace deductions with larger loans.)
If "everyone" lives on $X - Y% * $X for an entire year the loans are fully repaid each year and the government isn't "losing money" on its "loan terms" and is meeting the subsidies people expect from their government. That's not a "losing" situation. (It's not a likely scenario either, because at least some people are always going to want more than $X - Y% * $X dollars a year for their lifestyle or their dreams or their investments or their philanthropy or their vices.)
(ETA: It's also not directly an assumption in every form of UBI that Y% is less than 100%. There are UBI schools of thought that because of the time value of money, some charged interest is not only possible, but potentially a good idea as an incentive to invest the UBI payment in more than just mortgage/rent/lifestyle, but also something that does appreciate you with interest, such as the opportunity cost of accepting a job or a basic savings account. I tend towards the Y% <= 100% feelings, but I understand the Y% > 100% crowd.)