The proposals they are calling UBI are not even close. The whole point of UBI is that there are NO other govt social programs. NADA. No Social Security. No Medicaid. No food stamps. No subsidized housing. So in the US, you would take the ~$1.5T spent on social programs each year, divide it between ALL citizens (including babies - and maybe even fetuses), rich and poor, Bill Gates and the drunk in the ditch. That comes to about $5000 each.
Of course most people don't really need an extra $5K a yr, and Bill Gates certainly doesn't. The reason for distributing to EVERYONE is to avoid strings attached via qualifications. There is no penalty for getting an entry level job (as there is with welfare). This also minimizes the bureaucracy needed for the distribution. Basically, just send a check to every SSN for instance (not that I endorse SSN).
Before Biden, I could live on $5K/yr as a single, study in the library, scrounge computers from the dump to repair (which I do anyway), eat the Walmart rotissiere chicken + pack of greens for $5 each day (illustration only - change it up of course). Wear pants until they stand up in the corner, then get a new pair at the thrift. Sleep on the mulch bed at the mall (popular actually) - it is (or was) entirely doable.
From that basic subsistence, I could work on employment, write novels, start a micro-business, or otherwise be useful or at least amuse myself. The fear of statists is that some recipients of UBI might buy drugs and kill themselves - hence all the strings attached with food stamps, etc. True enough, but people manage to buy drugs/liquor anyway. A stupid objection.
There has always been a form of UBI - subsistence farming. Unlike UBI, this was hard physical work. But you could (and people did) live on it. The US has put many rules in the way of that, but there were a few dozen subsistence people living in parks that I knew about for the last 40 years - hunting, fishing, gathering, small plantings. Unlike tourists, they did not litter (nothing to litter with) - but the park rangers were always trying to evict them for some reason (don't want to give people ideas, I guess).
When employers are abusive to the extreme, subsistence farming has been the historical fallback. The problem with industrialization was that city dwellers had no such fallback - horrendous abuse must be endured, the only alternative was starvation. An English worker fleeing a city would find all the land owned by aristocrats, and any attempt to hunt/fish/gather would be punished as "poaching". This very real injustice is what gave rise to the very bad solutions of socialism and Marxism. G. K. Chesterton has many more insights in "Eugenics and Other Evils".
UBI is a libertarian socialist idea to replace subsistence farming in an industrialized world.
Of course, the actual programs called "UBI" being proposed are totally different. They are yet another redistribution of wealth on top of all the garbage already in place. Hence I do not support them at all. But I would support a real UBI that *replaces* existing programs at the same dollar amount. Compromise would be in not replacing everything. Maybe leave SS but replace food stamps with a UBI for example.