Benches on railing sides are put in after the railing, of the same materials. Note the decorative line in the back plate, the rounded foot of the legs, and the inset target blocks at regular intervals.

Using the angled corners for a bench is a popular idea, but not recommended  where kids can crawl on the bench and then over the rail. Unless of course, you had a privacy wall on that corner to block the view.

A good bench for open sides and low decks is the pedestal type. Note the minimal legging, which makes it easy to clear debris and snow from around them. Using a 6x6 post for the leg offers a great deal of strength and attachment area. The legs go through the deck and attach to the framing.