Is what banks do by giving students and recent graduates a sum of money with the university or work as collateral (60% cash and 40% purchases) considered usury, given that interest is calculated on cash withdrawals from the first day and on purchases after 45 days, and what is the expiation for those who have engaged in it if it is usury?

What you have mentioned is a form of usurious loan practiced by interest-based banks, and it is forbidden by Islamic law. Scholars have unanimously agreed that every loan that brings a benefit is usury (riba), and there is no difference between a monetary loan or an in-kind loan. Interest on all types of loans is forbidden usury, whether they are consumer loans or productive loans. Whoever is afflicted by such a loan must sincerely repent to Allah, and he must, out of necessity, repay the loan with the usurious interest if he is unable to repay only what he took, and he must resolve never to return to such an act.