I had two old aluminum window in my house replaced today with new retro-fitted vinyl windows. The window replacement person told me they had to re-trim around the windows as there was no original sheathing around them. They put trim around the windows and caulked around the upper and bottom edges of the trim. They just nailed the trim to the wood siding. I am worried that because there was no sheathing, there will be no way to really keep the weather from seeping through. Could they have done something else to ensure no moisture gets through?
Not sure what you mean. Many older homes do not have wood sheathing, but have something similar to drywall. It is designed for outer covering on a house, but does not hold nails very well. There are ways to insulate air gaps around windows, and insulating these is general practice when replacing windows. Although not all do it.