The joins are picked up based on the costing. The whole plan is built by costing different paths. If you have enough stats for a NUSI-UPI join you may expect Nested Join. Similarly, if the demographics are upto date, you may expect merge join for equality conditions (if one of the columns is a primary index). In this case, the rows are already distibuted in hash order. In this case, typically you might expect a merge join.
As posted earlier, you can not give any hints to force a specific plan.