UBI is implemented tomorrow. Every citizen gets $1000 per month.
Landlord now knows you have an extra $1000 that you never had before. Why wouldn’t the landlord raise prices?
Now you have an extra $1000 a month and instead of eating rice and beans for a few meals you go out to a restaurant. The restaurant owners know everyone is eating out more so why not raise prices and maximize shareholder profit as always. The restaurant/corporation is on TV saying, “well, demand increased and it is a simple Economic principle that prices had to increase. There’s nothing we can do about it”.
Your state/country has toll roads. The state needs money for its deficit. UBI is implemented and the state/country sees it as the perfect time to incrementally raise toll prices.
Next thing you know UBI is effectively gone because everything costs more and billionaires keep hitting higher and higher all time net worth records.
That’s the first part, but you have to keep going deeper to see how that leads to inflation.
If you open more restaurants, the market has more jobs for staffing. If the market has more jobs for staffing, then some businesses will increase pay to lure the better talent to their business. If businesses increase pay, then that can often get passed down to the customer.
San Francisco’s tech market was a good example of this theory in action.