Until the early 20th century, all glass was hand-blown. This video shows how they made large sheets back in the day. Invented in the 17th century, it was expensive and didn’t become the common method until the early 19th century. Something to keep in mind when writing a scene in which someone is seen in a window is that such glass had ripples and faults. The view would have been distorted to some extent.

Even more so before that, for all those centuries when window glass was crown bullions, or bullseye glass, as shown below. This video also shows the cylinder method.