Honestly, it depends on the bench. A lot of newer urban benches are designed with repair in mind, especially because city maintenance budgets hate replacing whole units over a loose bolt. If the bench is modular, the slats, frame, and legs are usually connected with standard bolts, threaded inserts, or captive nuts. That means a worker can tighten or swap a bolt in a few minutes without removing the bench from the pavement. You will often see stainless steel or coated hardware to prevent rust, and sometimes tamper-resistant heads to stop theft or vandalism.
But plenty of older or more sculptural benches are not that forgiving. Some have welded frames, buried anchors, or hidden fasteners that are hard to reach. If a bolt loosens there, the surrounding material may have shifted, the hole may be worn oval, or the bolt may be seized. In that case, a simple wobble might require drilling, re-tapping, or replacing a whole section. The real difference comes down to whether the designer allowed service access.
If you are checking a specific bench, look for visible bolts, accessible joints, and replaceable slats. Those are usually good signs that a loose bolt can be fixed quickly. Modern public furniture is moving in that direction, but not every bench is there yet.