Define the arrangement from inside and outside
Decide which sections will contain windows and how the pattern aligns with the home. Consider sightlines into the garage and how the space is used after dark. A photograph of a display door may show a different size or section layout from the product being quoted, so ask for a clear representation of the actual order.
Name the glazing rather than saying obscure glass
Write down the offered glazing option and discuss visibility, light and any required performance. Decorative inserts and glass construction are different choices. A pattern you like does not establish that it is available for every door model, size or required rating. Ask how the chosen option is represented in the product documents.
Treat later changes as a new compatibility question
Adding or changing windows after installation is not simply a styling decision. Keep the original door records and have the proposed work assessed for the assembly. Do not cut a steel section or exchange glazing on the assumption that an accessory kit makes every combination suitable. The replacement proposal should explain what changes and which existing components remain.
Your questions
Can the top section of every garage door receive windows later?
No. Suitability depends on the specific product and configuration. Have the manufacturer information and proposed change reviewed first.
Will a decorative insert guarantee privacy at night?
No. Pattern and privacy are different considerations. Discuss the actual glazing and lighting conditions, then review an appropriate sample if available.