There are many styles of built in benches that can be added as features to a deck. Search pictures online to explore a variety of design ideas. Most benches are framed out of 2x4, 2x6, and 4x4 material and covered with decking to create a solid surface to sit on that compliments the rest of the deck. Some benches use the rails as a backrest or may include one of its own.
There are no code regulations that enforce the construction of benches. Most of the attention will surround its appearance and how comfortable it is. A bench’s seat should be elevated about 16”-20” off the ground to provide a comfortable sitting position. Examine the heights and width of chairs and benches to get an idea of what dimensions you want to use for your benches. Make sure the benches will be functional. It isn't practical to have chairs placed in front of a built in bench. Remember that the bench needs to have proper support and should be solidly attached to the deck.