Take universal basic income, if combined with open borders, you are attracting people who would like to also get (say) $1000/m, who will now use that welfare to induce demand on people who may like to trade this freshly minted coin for some service they can render, so the economy grows through immigration instead of debt.
Without debt how would the state pay the monthly income for the increasing number of people coming to collect it?
So you have a bunch of new people move, they start collecting UBI, the government prints money to fund the new liabilities, the newly-immigrated spend their UBI, incomes/revenues go up for businesses, the overall size of the pie grows, and if inflation starts to get out of control you raise taxes such that everyone is still better off than before.
Unfortunately this wouldn't work because of politics. You need a relatively reactive monetary & tax policy which just is not realistic.
The money has to come from somewhere or you _will_ have runaway inflation.
Ok, why is that not true?
And I'd argue that any Einsteins born in Africa are going to find moving to another country and near impossible task regardless of how open the borders are.