Legally many things could happen, the lender could sue you, sell the debt to a collections agency (Possibly aggregated with many other small debts). They could call, mail or otherwise bother you to pay or they could report you to credit agencies.
However all these actions cost the lender expenses which they may or may not be able to recover from you. In the worst case (for you) they could sue you or send the debt to a collection agency who would then bill for you the debt itself and also their expenses for collecting it. These will sometimes be awarded by a court to the collector depending on circumstances.
More likely you will receive mail or robocalls demanding you pay, these could be as little as once a month or even every day if they are determined, but they may not bother if the debt is very small and the costs of doing so exceed it. They also could report you to the credit bureaus to damage your credit report, but this also costs them something.
However if the debt is only a few dollars it is also possible they will just write it off and not bother trying to collect. This doesn't make it legally go away immediately but it will eventually likely roll off due to statute of limitations rules for loans.