Lumber for doors

I’m starting to plan for a couple doors I need to make. One is a plain old 5-panel door (to replace my exterior basement door) and the other is a pair of garage doors, about 4 feet by 7 feet. (That will be an interesting if daunting project.) I’m planning on using Douglas fir, which is the most economical choice out here in Northern California. I’m wondering what kind of lumber I need to look for, for best and most durable results. Vertical grain only? Should I strive to avoid knots for the door framing? The doors will be painted, so it’s only a structural issue. (Is it even possible these days to avoid knots?) What grades should I be looking for? Anything else to consider in choosing the wood?

Also, just a side question: the 5-panel door I’m replacing is vintage, i.e., “legacy,” i.e., old. The panels were solid wood. I suppose it’s easiest to make the new panels with plywood, but how did they make these panels before? How thick? Did they typically just cut them from wide pieces of wood? They never appear to have been made from smaller widths glued together, but that would have been an awfully lot of wide panels to cut.