I believe what we have up there is two layers of plywood, one the actual sheathing and the other the soffit material. The soffit material could also be t&g boards (more expensive) It should have a sealer or clear finish of some kind (maybe Cabots Clear), but the eaves are fairly protected.

Thanks so much for the education. Appreciate your time and thoughtful response.

What kind of window framing system is used here?

As I recall, there is a steel flitch plate in the header,

but the verticles are paralams.