First of all, there is no one idea of guaranteed basic income. There are various models which disagree regarding implementation details and lead to completely different outcomes (How high exactly? Does it include children? How about the different types of foreigners? What do we do with all the pension funds people paid into for decades?).
But many ideas are based on the idea that all forms of welfare spending are abolished and replaced with a fixed monthly payment which covers the cost of living a decent human life (with different interpretations of "decent").
That means instead of our very differentiated and complicated welfare system where everyone might or might not be entitled to dozens of different forms of transfer payment depending on countless factors, everyone receives the same amount of transfer payment from the government. Our welfare system was carefully calibrated over decades with the intention to give everyone exactly as much money as they need to survive. How well that works is a topic for a discussion which doesn't belong here, but for sake of argument let's just agree that this mostly works out. Basic income replaces all that complexity with a simple "everyone gets the same" system, so unless you want a minority of people who already have it bad to live an even worse life than before, the overall payout must be higher than before. That's just basic math.
So how can a basic income be financed?
Remember that the average working citizen now has a basic income plus their normal wage. That means it is now possible to increase taxation in a way that their actual disposable income is reduced back to what it was before the introduction of basic income. There are different ideas how that can be done, like increasing income tax, increasing value-added tax, increase tax on employers in exchange for reducing employee protection laws which are now no longer required, or a combination of these.
Most of these calculations assume that the amount of working population and their productivity stays the same. It is hard to predict how basic income will affect employment numbers. Critics of basic income argue that fewer people will work when they don't have to. Proponents, however, predict that even more people will work, because the current welfare system actively discourages welfare receivers from taking small jobs (if you make less money per month than you get welfare, that money is subtracted from welfare, so you end up with the same amount of income whether you work or not) and that the reason for unemployment is rather lack of jobs than lack of people's motivation to take them. Who of them is right is unfortunately impossible to predict.