Exterior seam molding source needed

Thanks for that link, but looking at these I can't figure out what most of them are for :? (and of course no description as to their purpose, lol) although I suppose you're supposed to know what you're looking for in the first place. I could be completely off base, but what I thought I need is a piece that essentially has a double-sided u-shape so both pieces of plywood can fit into each channel, but maybe I'm thinking of stuff you'd use in a tub enclosure. If you could point out which piece serves that purpose I'd really appreciate it, and thanks again for your help ;?)