UBI isn't going to be a fat allowance to fund a comfortable life of leisure. It's going to cover subsistence level costs only. Many, many people work second jobs in order to be more comfortable. I don't think it's far-fetched to expect that most people would work a first job.

That likely depends on the individual proponent of UBI. From what I hear in Germany, it's supposed to be higher than our current from of welfare, which puts it somewhere at ~1000 Euros a months at minimum, so you can cover an average sized apartment, subsidized health insurance, utilities etc.

I know people who live -in their eyes- comfortably off of that. They see no reason to change anything, as enough is provided to eat, sleep, smoke, get drunk, have a TV and an xbox, a computer and internet and use public transportation to meet with friends. Sure, you can't easily travel, and you're not going to buy a house any time soon (but if you own one already, you can keep it; if you're still paying the mortgage, the state will cover it for you within reasonable limits), but it's a comfortable life if you don't have a lot of ambition or can make some money on the side.

Lots of people do that. Or more commonly they pick up extra shifts/OT at their main job. People will still work if they have their essentials covered to buy a new car/go on vacation/ etc.

Ah, I wasn't aware that's a thing in the US. It's virtually unheard of for employees in Germany, where a very strong social safety net exists.