As I look at this relatively small and simple frame, I'm reminded that even a small timber project tends to have a variety of joints. In this one I'm counting at least 8.
This next joint forms the outside bottom corners of the frame. The 2 sills come together in a slip joint, or open mortice and tenon
and the corner post comes down with a tenon into a mortice in the middle of the slip joint. The post sits in a 1/2" deep "housing",
which helps to keep it in place and straight, and the tenon from the corner post locks the slip joint together. Here's a drawing that should show you what's going on.