These large 2 by 8 boards attach to the house or an adjoining solid structure. They provide support for the beams or joists. These provide support for the joists. When you build an attached deck, one beam will sit opposite the ledger. For a freestanding deck, this beam will take the place of a ledger.
Beams can sit directly on a post or can fit into the post.
They are usually built of solid lumber - 4x6s or 4x8s. 6. These provide support for the decking and allow you to lay decking that wouldn't span from beam to beam. These provide added support for tall posts and deck substructures. Check building codes for bracing and blocking requirements. The pattern you choose for decking will affect the substructure and the overall look of the deck.