Rob, the seat blanks are pine. They are not glued up. I went to my Amish sawyer acquaintance and asked if he could find me a board about 2.25" thick, 20" wide, and 10' long. He did. It was green and roughsawn when I put it up in my attic a year ago, after crosscutting it into suitably long blanks and sealing the ends. After air drying for a year, the moisture content is around 8% or so. Before I start shaping the seat, I flatten one face, then plane the other face parallel to it at 1 7/8" thickness. The pine is nice to work. Many chairmakers glue up the seat blank, but I like making it from a single board, and that's how Elia taught me.
Windsor chairs are typically made of a few different woods. The legs are maple, the spindles and continuous arm is white [or red] oak, or ash, and the seat is a soft wood, usually pine, but could be poplar. The chairs are usually painted, alleviating any funny looking wood combo appearance.