Fair warning: I’m quite ignorant in terms of economics, so this is a naïve way of looking at it.

The question that always pops up for me when it comes to UBI applied to the current capitalist system: even if you did actually come up with the money somehow (which is a pretty huge if as you say), once everyone has X “base money” per month, doesn’t that mean the cost of living (specifically renting) will rise to match this new “base”?

The cost of living would certainly rise somewhat but the point is that UBI is redistributive: the same absolute amount to everyone raises low incomes by a larger percentage than high incomes. Long term effects are hard to predict but in the short term it would mean the poor doing slightly better while the middle class is slightly worse off. The non-working (owning) class would be mostly unaffected as assets are insulated from inflation.

Another factor to consider is that putting more money in the hands of people in need of <thing> means producing <thing> becomes more profitable and thus more investment and resources are directed towards <thing>. If we assume the economy works the way the proponents of capitalism say it does, this should eventually drive the cost of living back down.

But personally I think the biggest benefit of UBI would be the reduction in number of people who are desperate enough to accept work – both legal and illegal – that is unfairly compensated, inhumane and/or immoral. The existence of that class of people is the driving force behind many societal problems. Exorbitant amounts of resources are wasted treating the symptoms of those problems instead of fixing the root cause.