I don't think you (And a fair few others) understand the concept at all.
Very very basically:
Everyone gets a basic income that is enough to reasonably live on (not like out of work benefits now which are calculated to be barely enough to punish people into going to work).
Everyone is also free to earn extra money by going to work if they want to.
The models I have seen explained do offer an extra sum to those who cannot work due to disability/long term illness.
So AI Coder gets her UBI and she gets her wage for going to work each day.
Stacey sat on her fat arse at home just gets her UBI.
There would undoubtedly be people who sit on their arses at home, but currently those people who do not want to work, are insufficiently intelligent/motivated/educated to be employable... are actually costing the taxpayer more money on being chased into work, sanctioned for whatever, getting into debts they cannot pay, than it would cost to just give them enough to live on and leave them alone.
And for all the 'sitting on their arses' people, there will be a similar number of 'can work a bit but not enough to fulfill current benefits criteria/earning requirements' who WOULD do something if they had the freedom and security to do it.
If people had the freedom to work part time, volunteer, try out starting up a business or self employment without the threat of losing everything, the economy would be better off.
If people had enough to live on, spending is likely to go up - good for the economy.
If people are not scrabbling to survive, then zero hours contracts and shitty toxic work environments and attitudes will be a thing of the past. Employers will have to work hard to get employees, we won't have people doing meaningless jobs and mega layers of management, things will be streamlined because labour will not be cheap.
We might see a return to more manufacturing, more 'lifestyle' jobs (because people can take a break from it, its not so precarious with the back up of the UBI), industry the UK has lost over the last 100 years, that it now needs due to Brexit.