A: Composite deck lumber never needs finishing, but it can only be used on the top of the deck where you walk. Structural elements still need to be wood, and how posts and beams and joists go together affects rot resistance of a deck to a significant degree.
The main thing to understand is that trapped moisture within the structure promotes rot. This is why it’s vital to design your deck to reduce wood-to-wood contact areas as much as possible. Almost no professional deck builders follow these principles.
Will your design include beams? Most do. Use solid 6×6 or 8×8 pressure-treated timbers instead of spiking together several 2x8s, as is usually done. Also, wherever deck boards meet end-to-end, install two joists side-by-side with a 1” space between them. This allows you to keep the ends of deck boards 1/8” apart, so water trickles away instead of being held between vulnerable end grain surfaces, promoting rot. Details like these are important enough that I’ve written a video ebook on the topic. It also includes deck finishing strategies that actually work. I call it Love Your Deck, and you can check it out here.