The short answer is it depends on the bench, but most street benches require at least some assembly. Unless you order a fully assembled unit from a commercial supplier with delivery and installation included, you will usually need basic tools to bolt the seat slats to the end frames or attach the legs. I have put together enough park and sidewalk benches to tell you it is rarely difficult, but it is much easier with a second pair of hands.
For classic cast iron or steel benches with wood or metal slats, the seat and back slats typically arrive separate from the two end frames. You will need a socket wrench or adjustable wrench, and often a hex key. Some models use security or tamper-resistant bolts, which may require a special bit. Recycled plastic benches are similar: the frames may be pre-assembled, but you secure the slats yourself. Concrete and stone benches are the exception. They are often delivered as one heavy piece, or as a base and separate top, and you may need lifting straps and a pry bar rather than a hand tool.
What tools should you have ready? A socket set, combination wrenches, hex keys, a rubber mallet, a level, and a cordless drill or impact driver with the right bits. If the bench will be installed in a public area, you may also need anchor bolts, a hammer drill, and a masonry bit to secure it to concrete. Check the product description for some assembly required or fully assembled. If it says knocked down or KD, that means the bench ships flat and you will put the major parts together.
Start by laying out all hardware and reading the instructions. Attach the slats loosely first so the frame stays square and you can adjust everything. Then tighten from the center outward, check that the bench sits level, and only then anchor it to the ground. Do not overtighten wood slats on a cold day because the wood expands and contracts with weather.
So, do street benches come pre-assembled? Some do, especially residential-style garden benches and certain commercial models with white glove delivery service. But the typical heavy-duty street bench is not fully assembled out of the box. Plan on spending 30 to 60 minutes with basic hand tools. If you would rather not deal with tools at all, look for suppliers that offer professional installation.