Honestly, the amount of assembly for an outdoor bench varies quite a bit depending on the brand, material, and price point. Most benches you order online or pick up from a big box store arrive flat-packed in a box. That means some assembly is almost always required, but it is rarely difficult. In my experience, you are usually looking at 15 to 45 minutes of work with basic hand tools.
A typical wooden or metal outdoor bench will come with the seat, two side leg panels, a backrest, and a bag of bolts, washers, and nuts. The instructions are usually a single sheet with diagrams. You basically line up the pre-drilled holes and screw everything together. It is a one-person job for most models, though having a second pair of hands can help hold the backrest steady while you tighten the first few bolts.
Now, the big question: does it come with tools? The answer is often yes, but only a small Allen key or hex wrench. Many flat-packed benches include the exact hex key needed for the bolts. Some kits might also include a small open-end wrench. However, if the bench requires a Phillips screwdriver or a larger socket wrench, you will likely need to provide that yourself. A power drill with the right bit can speed things up dramatically, but I always recommend finishing by hand so you do not strip the wood or overtighten the metal.
Cheaper benches with plastic or resin components sometimes snap together with no tools at all, which is great if you hate hardware. Higher-end teak or cast aluminum benches may arrive partially assembled, with just the legs to attach. In rare cases, a bench arrives fully assembled if you buy from a local furniture store that offers delivery and setup, but for online orders, expect some DIY.
Before you start, check the package for a small plastic bag taped to the inside of the box. That is usually where the hardware and the included tool hide. Lay out all the pieces and count the bolts before you begin. Use a soft surface like cardboard or a blanket to protect the finish. And do not tighten everything all the way until the whole bench is loosely assembled—this lets you align the legs and backrest without fighting the frame.
So, to sum it up: yes, you will probably need to assemble your outdoor bench, but it is straightforward. An Allen key is usually included, and you may only need a screwdriver from your own toolbox. Budget about half an hour, follow the diagrams, and you will be sitting comfortably in no time.