A lot of it will come from being able to remove services like food stamps, unemployment benefits, various pension and social security payments etc. The rest will probably have to come from raising taxes. You can structure the UBI and tax rises in such a way that they end up cancelling out for those people who don't 'need' UBI
Yea, it didn't mean that UBI could replace all of social security for everybody. Social security is (greatly simplified) made up of a guaranteed minimum payment + a payment you've 'earned' by working. UBI would replace (and be equal to) the minimum payment, the money that you've 'earned' from working will still be paid out. The core point is that at least a chunk of UBI will be replacing existing government subsidies and not be in addition to them.
You have social security recipients who have earned (and are eligible for and counting on) benefits of $3K, $4K, or $5K per month. Removing those and replacing with $1K/mo of UBI will be...difficult...